Hey Mark,
FCPX doesn’t have a H.264 workflow, other than transcoding it to Apple Pro Res.
ProRes, is much less compressed than DSLR H.264 files. So converting to ProRes and back to H.264 won’t make a difference. The postproduction companies I work for (FCP 7 most of them), here in Berlin, use this workflow all the time, no quality loss.
Now, the benefit of using Premiere with H.264 is that it does it natively. So no need for transcoding. You do save some time and a lot of extra HDD space.
Personally I don’t use Premiere at all, since most companies I work for here use either Avid of FCP 7. I really like what Apple did with FCP X. I don’t really care if I need to transcode H.264 to Apple ProRes and need more space. But that’s just my workflow.
Cheers,
Nik Bäurle