What people do not realize is that FCP 7 was the 7th version of a totally different program.
I actually think this here is one of the threads that acknowledges the awesomeness of FCPX in many posts. Everyone (except for one poster) is using it and embraces it.
And you are right that many things are not only different (which scares away some formers users of FCP) but better than in previous versions. One of those things are the export abilities of Motion.
However, a roundtrip possibility would still be a very desirable feature. The first time I did a Greenscreen composition, I was annoyed about having to export back and forth, because Motion's keyer is far superior. Well, in the latest update they included this magnificent keyer directly in FCPX, but still I find myself many times wanting to e.g. add some visual effects to a short clip, do a more complicated masking or rotoscoping, or motion track and match something inside my videos.
You are correct that for titles, transitions and such, the way Motion and FCPX now work together is brilliant. But for visual effects, a roundtrip feature would make life much easier.
By the way, I am also very impressed by how quickly and thoroughly Apple adds new features to this rewritten editing suite. I have edited with FCP7 in my university studies, but would never have gathered the money to buy it. The new price tag is really unbeatable and made me upgrade from FCE overnight, no looking back.
Greetings from Germany and lighten up,
Floh