But they remain jedi. You can go with Knight and go Dark Side. That's your call. Implenting defection does nothing good to the game. You can play whatever you want and it affects your story. Just to visit other faction fleet, I dunno, not worth wasting time.
For example WoW has faction transfers yeah, but it doesn't affect anything at all because:
1) There's no storyline.
2) Classes are not exclusive to the faction. Everyone can be Hunter either Horde or Alliance.
Problem with SWTOR is that each faction has EXCLUSIVE classes, with other abilities and so. I don't wanna see IMPERIAL agent running around on Repub. fleet. I can make my own Imperial Agent and go full re*d*ard mode and go LS or something, playing what I want, but I still be Imperial because it makes sense.
My point is defection is good for some NPCs to twist story or something, but not worth wasting time because some people are too lazy to level mirror characters. Gunslinger is exclusive to Repub - 2 pistols etc. Sniper is exclusive to Imps and has sniper rifle obviously, making him special and somewhat different to Slinger. Put time in defection and you lose your personality.