90% of all strum guitar VSTis
Ample, U-Jam ... Most of them.
Multi layer synths / sequencers / arpeggiators, such as Halion, Omnisphere... pretty much all of them
Virtual bassists, Virtual orchestral instruments, virtual pianists (ToonTrack Ezkeys, EZbass and many others)
List goes on.
While BIAB has a very good range of styles, VSTi's can bridge pretty much anything that is missing. To make things work properly in BIAB we still need:
Fixing bug mentioned in this post.
VST3 support (Patiently waiting....)
Being able to add keyswitches (midi editing), without overwriting them on the next regeneration.