Adding my 2 credits as a person who utterly hates level sync: I had a quest on Ilum earlier. At level 65, it should have been nice and easy - run to the objective, talk to the guy, leave. Instead, what should have taken two minutes ended up taking ten as several groups of enemies spotted me, aggroed, pulled me backwards, and stunned me. It wasn't fun, it wasn't exciting, it was annoying.
Sure, leave it as an option. But make it OPTIONAL. Maybe one day I'll want to do a Korriban heroic and have it be challenging. But more likely I'd want to hunt for datacrons without having to fight nonstop, or get a kill achievement without the fights sapping my will to live, or just go sightseeing and enjoy the scenery the developers worked so hard on without being interrupted by obnoxious NPCs I couldn't care less about. Plus I miss being able to go back and solo earlier flashpoints...few things in this game are more satisfying than oneshotting the boss who killed you a dozen times earlier.
If you're concerned about PvP, sure, turn on level sync in pvp scenarios, I don't care. But it should never have been a mandatory feature in solo PvE.