The MIDI that comes with the MIDI packs is different from the MIDI that comes with the instrument packs.
The EZKeys MIDI library for the Small Upright and the Upright Piano (the two instrument packs I own) appears to be the same...samples from many different genres.
Each MIDI expansion pack provides a large number of MIDI files specific to the pack's genre. For example, the Blues MIDI pack is segmented into Straight 4/4 (Intro, Verse, Chorus), Swing 4/4, Straight 6/8 (Intro, Verse, Pre Chorus, Chorus, Bridge) and Swing 6/8 (Intro, Verse, Chorus). Swing 4/4 has seven 'songs' in it, each with Intro, Verse, Chorus and Bridge. Each subfolder has a number of variations in it.
Unless you plan to use the audio output of EZKeys and not just the MIDI, I see no reason to buy any additional instrument packs. I got the Upright Piano because I didn't much like listening to the Small Upright that was the bundle I bought. I use the MIDI to feed other keyboard instruments.