Based off of how he acted in the latest episode, his motive to join the Rebellion doesn't seem to be out of revenge.
I think that the Ghost crew, well at least Ezra and Kanan, will ultimately
die at the end of the series. This is the quickest and simplest way to make sure fans don't ponder on why Luke is the only Jedi that's on the Rebel's side in the original trilogy, since the show's cast is obviously on their team. It would also make what Wedge says in Aftermath
make a little more sense, since he will probably stay with Phoenix Squadron for this season and beyond, allowing him to bond with the crew.
Another possible ending to the series is their exile or capture, but who knows? This is all just speculation for a show that has another good season or two left in it.