In Addictive keys, you can use change the temperments of the keyboards. Each Keyboard by default takes the temperament of the original recorded instrument, which are sharper in the higher ranges.  There is a drop down box they provide a whole range of different temperaments, about 30 in total.  Top of the list is equal. In a midi environment where all the midi will generally default to equal, why wouldn't you use use Equal over the recorded or the other options?
 

Tuning and MIDI are separate things, and, depending on the instrument, may be wholly unrelated. 
 
MIDI has no knowledge of what an instrument will do with the commands it's given. It might transpose the note, or even play a non-pitched sound effect. It's entirely up to the instrument.
 
As for why you might choose an alternate tuning, that's too esoteric a question for this old school rocker. Maybe if you were into Arabic folk music?
 
 

Wo wo, don't get the Arabic Folk music crowd all riled up! The next thing you know they are crying about back in '65 when Aziz Zimmerman played an electric Shlasma at the Ben Ghazi Folk Festival. Still pretty raw. I had a friend who was at the BGF Festival a few years ago and said the Weavers were the closing act. I said I thought they were all dead by now. He said they were. It wasn't a very good show.

LOL! No encores.

No encores. Just the crowd gently humming Good Night Irene.

Although there are about thirty, it's the Equal Temperament vs Originally recorded that I am trying to get my head around. Especially as the default is not Equal temperament.
 
I had read a while back the piano tuners detune the higher range up a little, but forgot the reason for this. This isn't the same as using eastern tunings, where say every 6th is flatened, it relates to equal temperament.  But when I'll be using synths, samples and acoustic instruments that aren't raised in the upper ranges, I don't know why this would be the default for addictive keys. This is more about intonation than funky turnings.
 
Each of the Trio have their own intonations, as 'originally recorded' is different for each original instrument and they all have their own intonations depending who tuned them.  

Ok, so it''s called stretch Tuning.
 
So is Stretch tuning really intended for solo use? Or does it not mater in an orchestral setting due to string, brass and wind players able to make adjustments to their instruments, and adjust finger positions (string) or Embouchure (Wind and Brass)as they play.
 
In the Midi world, we don't make these adjustments, it's equal temperament that hasn't been stretched. So should the default for Addictive Keys be changed from 'originally recorded' to 'Equal temperament' when used with larger midi ensembles?
 
I'm more after opinions than fact
 
From Wiki

'Stretch

The tuning described by the above beating plan provides a good approximation of equal temperament across the range of the temperament octave. If extended further, however, the actual tuning of the instrument becomes increasingly inaccurate because of inharmonicity, which causes harmonics to run slightly sharp, as increasingly higher tones in the harmonic series are reached. This problem is mitigated by "stretching" the octaves as one tunes above (and to an extent below) the temperament region. When octaves are stretched, they are tuned, not to the lowest coincidental overtone (second partial) of the note below, but to a higher one (often the 4th partial). This widens all intervals equally, thereby maintaining intervallic and tonal consistency.
All western music, but western classical literature in particular, requires this deviation from the theoretical equal temperament because the music is rarely played within a single octave. A pianist constantly plays notes spread over three and four octaves, so it is critical that the mid and upper range of the treble be stretched to conform to the inharmonic overtones of the lower registers. Since the stretch of octaves is perceived and not measured, the tuner is aware of which octave needs "more" or "less" stretching. A good tuning requires compromise between tonal brilliance, intonation and an awareness of gradation of tone through the compass of the instrument. The amount of stretching necessary to achieve this is a function of string scaling, a complex determination based on the string's tension, length, and diameter.
In small pianos the inharmonicity is so extreme that establishing a stretch based on a triple octave makes the single octaves beat noticeably, and the wide, fast beating intervals in the upper treble—especially Major 17ths—beat wildly. Of a necessity the tuner will attempt to limit the stretch. In large pianos like concert grands, less inharmonicity allows a complete string stretch without negatively affecting close octaves and other intervals. So while it may be true that the smaller piano receives a greater stretch relative to the fundamental pitch, only the concert grand's octaves can be fully widened so that triple octaves are beatless. This contributes to the response, brilliance and "singing" quality that concert grands offer.
A benefit of stretching octaves is the correction of dissonance that equal temperament imparts to the perfect fifth. Without octave stretching, the slow, nearly imperceptible beating of fifths in the temperament region (ranging from little more than one beat every two seconds to about one per second) would double each ascending octave. At the top of the keyboard, then, the theoretically (and ideally) pure fifth would be beating more than eight times per second. Modern western ears easily tolerate fast beating in non-just intervals (seconds and sevenths, thirds and sixths), but not in perfect octaves or fifths. Happily for pianists, the string stretch that accommodates inharmonicity on a concert grand also nearly exactly mitigates the accumulation of dissonance in the perfect fifth.
Other factors, physical and psychoacoustic, affect the tuner's ability to achieve a temperament. Among physical factors are inharmonic effects due to soundboard resonance in the bass strings, poorly manufactured strings, or peculiarities that can cause "false beats" (false because they are unrelated to the manipulation of beats during tuning). The principal psychoacoustic factor is that the human ear tends to perceive the higher notes as being flat when compared to those in the midrange. Stretching the tuning to account for string inharmonicity is often not sufficient to overcome this phenomenon, so piano tuners may stretch the top octave or so of the piano even more.'

Now that's a bit of a stretch. Better hum "Come on Eileen" like poor old Johnny Ray.

Here's a good demonstration of just vs. equal temperament. It illustrates why electronic music sounds fundamentally different (and to many, less-interesting) than traditional acoustical music.
 

Kamikaze
Although there are about thirty, it's the Equal Temperament vs Originally recorded that I am trying to get my head around. Especially as the default is not Equal temperament.
 

My guess would be that someone would want the "originally recorded" to add the imperfections that were inherent in the classic instruments.  They would give you Equal Temperament as an option in case you wanted the perfect pitch.  As I understand it, no piano can be perfectly tuned across all the keys (although i don't see the reason).
 
I'm not familiar with the instruments in Addictive Keys, but it seems similar in this respect to amplifier simulators.  I appreciate that tube-driven Amp Sims will distort as you increase the volume (similar to Originally Recorded) instead of just getting louder (Equal Temperament).  It was originally a flaw, but one that we have come to want.