While Adobe and Apple each link their editing software with their own respective compositing software, saying 'AE is to Premiere as Motion is to FCPX' glosses over the strengths and differences of each package.
Sure, AE is a powerful compositing app, and it's dynamic link to Premiere is convenient, but Adobe is still playing catch up to Apple when it comes to application interoperability.
Say, for instance, you'd like to make a transition between two clips, and you'd like this transition to be a stylistic element of your production which you can re-use as often as you like. This is something at which Motion excels and at which AE struggles.
After Effects is more powerful when it comes to a lot of VFX work, and its scriptability is a powerful feature which is unmatched in Motion. However, I would argue that the "bond" between FCPX and Motion is stronger than the one between Premiere and AE. But when you're talking about using FCPX with AE, there is NO link and you have to render bi-directionally.