On-Screen keyboards take up valuable screen real-estate on already-limited devices.
Emacs with a virtual keyboard is a nightmare.
OTOH, if the physical keyboard lacks Meta Alt and Super keys it's pointless.
OTOOH, virtual keyboards allow for so many variations that are hard to do with physical keyboards. Leaving the ergonomics of typing on glass aside, you can have ergo shape-forms, bizarre key-chord scenarios, dvorak layouts, etc etc etc. it's virtual, man!