There is nothing you can do in Motion that makes it worthwhile to export any video from FCPX. Everything you can do in Motion can be *made available* in FCPX (... well... almost everything...).
You have Effects, Generators, Titles and Transitions to pick from. If you are considering making alterations to the video itself inside motion, consider starting with an Effect Project. The Effect Source layer you are given is exactly the content of the video of the clip in FCPX... It is a "holder" for the video in the Storyline. When you apply an effect to a clip in FCPX, it is *exactly* as if you exported that clip to Motion, performed your magic on it and sent it back, with one exception: you can use that same effect over and over and over again in the future with any other clip in FCPX... forever (or for as long as Apple produces FCPX...)
If you decide to use a Title, then the Title Background is everything that is visible under the title added to the storyline... And the Transition is exactly the handle lengths of the incoming video clip and the outgoing video clip in the storyline with Transition A (green layer) and Transition B (red layer) respectively.
[Generators are like "old" Motion projects, and they have no "reference" to clips inside FCPX at all.]
These "holders" (they look very similar to Drop Zones) stand for the actual clip in FCPX. There is no need to export it, no need edit it in Motion, and no need to roundtrip it back into the project. Pick your project type, build whatever actions you need, and Save... Then use the effect inside FCPX just like you would any other pre-built effect.
Even while developing an effect, Saving the Effect project is immediately available in FCPX, so it can be tested and tweaked *with* the video you mean to have altered.
Roundtripping was a huge waste of time. The Motion "engine" is built into FCPX. That means once you've built the template or even just a simple project, you have the power and flexibility to make changes directly within FCPX (provided you remember to publish the parameters you will need to have access to in the future 🙂 )