Well, if you really must have an explanation: There's an infinite number of 3D programs, CAD tools, rapid prototyping and design apps, interior decorating/ architectural/ garden and landscape planning and whatnot and most of them use proprietary formats. If Adobe wnated to support them all, it would take them a lifetime. That's what conversion tools are for. If you get my meaning: If you stasrt supporting C4D, MAX and Maya users will be the next ones to scream. It's an endless cycle. PS supports OBJ, Collada and 3DS which most apps provide export options for, which is usually good enough. If one they they support Alembic and a few other formats that's certainly okay, but I wouldn't go crazy over it. I agree, though, on U3D - it's useless. Nobody out there ever uses it.
Mylenium
