The TIFF absence is probably an oversite, like the earlier omission of OBJ-import. But remember, this is the PC version of P5; the PC version of P4 doesn't have QuickTime support either.

PNG is a perfectably acceptable image format for compositing; it has alpha-channel support. I use it all the time. It compresses better than TIFF, also.

My copy of Poser 4 (with Pro Pack) even exports to PhotoShop format, although I see this feature isn't mentioned in the official listings.

You can always export and then convert to the desired format. On the PC, QuickTime Pro will handle your AVI-to-MOV needs. And Paint Shop Pro can batch-convert an entire directory of PNGs to TIFFs.