2 Sets of Monitors

I have seen many studios with 2 sets of monitors. My current monitor set up is I have a pair of 1st gen (purchased in 2004) Event 20/20s (passive) hooked up to a crown XLS 2502 amplifier. My question is would anyone recommend getting another pair of studio monitors and if so why? Also if so what would you recommend? Thank you

In the big studios the standard set up is a big set of full range speakers hung on the front wall. Then you might see as much as 2 even 3 sets of near fields. The offering is because different people will be using the studio and everyone has a favourite. The studio needs to please the status quo.  The most common speaker siting there has always been the NMS10. Auratones are also a very common near-field. Now of course powered speakers are the norm and you'll see more of a variety. 
 
In a home studio there's not much point in 2 good quality near-fields. You really need to learn your monitors and having 2 sets will just water down your ability to learn and trust your monitors. 
Example, I have my NMS10's and a set of Tannoy PBM 6.5's. If you hook them both up the Tannoy's would win the overall sonic impact and full range sound shoot out hands down. But..my mixes are always accurate when I use the Yamaha's.
 
I used the Tannoy's for my first 10 years of recording and back then I made pretty good mixes. But it always took 6+ times from the car back to the studio to accomplish this. Within a few weeks of using the Yamaha's all that changed and I often get it right the first time. 
I still use the Tannoy's to listen to mixes but they are now in another room serving duty as a home stereo set.
 
I do have a second set of monitors rigged in the studio which are the equivalent of a computer speaker. I can A/B a mix with those but I never really learn much.  I also have a Sub which will make up for the Yamaha's pathetic low end. I only turn it on AFTER I'm happy with a mix. It certainly sounds better but is a dangerous tool to use full time while mixing.  
 
So bottom line, you can always purchase a "better" set of monitors and retire your old ones. But if you are happy with your mixes why spend the money. If your not happy with your mixes then yes, buy a new set but then why would you want to use the old ones?  Sell them and buy a nice chair.  
 

Two sets of near fields are also fine indeed as long as I mentioned they are different enough from each other.  In my case the Mackie HR824's and the Yamaha HS7's sound very different to each other.  So in that case the second set is still valuable and useful.
 
What would not be good is for the second near field set to be virtually identical in sound to your main speakers. For example there are some near fields that have a very close sound to the Mackie's and then you would not be gaining much. You can still do it on one set of speakers too but having the other is a bit of a luxury. 
 

For home studios most of people simply use the mains and then drag copies to other environments.  I listen to mixes on this (non music) computer's speakers, my jeep and living room book shelves.  But most pro studios offer near fields for their clients to also reference, while some engineers use their nears for most mixing and use the mains for check up.  As noted above, mains can give someone too good of a sound (when most users are listening on ear buds) and too loud when listening all day.
 
The little yamahas - msp5s, are often used for near fields.  Those would be my choice in the lower price range. Their HS series were designed for home use and pleasure rather than to be "revealing" (tho they are fine as alternatives).  But really, listening to real world alternatives (like in a car) will give you a good idea of product flaws and, unless you have a well-treated room, environments that don't share the same flaws despite using different speakers.
 
@ 
 
 

Seems just having different speakers may not solve that issue. You still need to listen in different "spaces" 

In a few more years earbuds will be the standard 

I'm considering adding a 2nd pair myself.
 
A few years ago I tried out the Yamaha HS5 or HS50 (I'm not sure which) in a store. At the time I was unimpressed. I thought that they sounded "unmusical", i.e. the music playing through them was just a succession of sounds. There was no "groove". It was like it was not held together as an integrated whole. Listening was not an enjoyable experience.
 
But when I thought about it afterwards, I realised that this is exactly what I need for critical listening. When I listen back to to my projects, I often get distracted by the groove of the music or drawn in by the atmosphere, or maybe start focusing on phrasing and counterpoint. I end up forgetting to listen critically. This doesn't help when you are trying to solve problems and focus on particular issues. But taking away the musical aspect forces me to focus on things like sound and balance. The Yamahas bring out the worst in a mix and this is a good thing. I would therefore definitely go for the Yamahas as a second monitoring system.

I carry my music to a variety of locations and systems on CDs. It isn't fast but it can be revealing.

Speaker location in the room also matters.

I'm not trying to say my way is genius or best. We all balance budget and needs. I'm only saying I have not felt the need for a second pair of speakers in my mixing space to be a priority over a nice Mic.

Interestingly it seems the majority of companys have shifted to the active monitors. I understand why to some extent buy now wondering if the passive setup is inferior or just more burdensome.