There's certainly no problem in people rendering the models / animations out for use in other projects, but as stated, the file space taken up by the 3d files is comparatively small, especially when considering that "adding a gun" involves merely adding an extra .x file and texture, rather than adding new frames of animation for all possible skin, gender, clothing and hair combinations.
By way of a very simplified diagram, if we think of :
Gender : male / female
Skin tone : black / white / brown
Clothing : red / yellow / green
Weapon : gun / knife / hammer
In 3d, that is seven models (body x2, clothing x2, weapon x3) and 12 textures. In sprites, it is a full set of all animations for 54 different character models. If we were to then add for example two different "hat" options, that'd be 9 models and 14 textures, or 162 full sets of sprite animations.
Then if we decide to add an alternate trouser colour (say blue for example), that's 9 models and 16 textures, or 324 full sets of sprite animations. Then we add another weapon, 10 models and 17 textures, or 648... you see how sooner or later it's going to go a bit mental - and looking at the file sizes of existing sprites, we're perhaps looking at 3GB of sprite files, or about 15MB of 3d stuff.
Of course, this only applies for "maximum customisation with all models, textures and objects interchangeable", where unless a "layered sprite" system (like FO tactics) is used, then 3D is the only option. However, for smaller projects which require a couple of extra human models (i.e. the little girl model for example) then rendering the sprites is definitely a good idea. If someone was able to set up some template files for all animations and all directions, I think it'd certainly be useful for the modding community as a whole.
