I would imagine it saves a lot of memory sticking with 16bit although photo viewing would be nice in 24bit, although probably only noticeable when extremely zoomed in to a photo.

Wouldn't 24bit also be a hit on the 3D acceleration? I am not sure if its possible to switch between 16bit (running a game) and 24bit, with compositing enabled. Could be a reason to keep Maemo 16bit all round.