Online is very, VERY useful. Playing a Good player online is much more useful than playing a crappy player offline.
The distinction is basically SETUPS!! Even with 2f or 20f lag, if its consistent it can be dealt with. So basically you are required to adapt and anticipate that much more ahead, and be able to read your opponent that much better. It is not a better way to play the game, but it is like a sort of "hyperbolic time chamber" in that you are dealing with situations you wouldnt normally have to deal with.
ie. Whoahhzz live in EC, Suirad in Florida. When I play these guys, there is no amount of lag tactics, not even with throws. Both players and myself play heavily "SETUP" mind set, so if I get hit by something deemed a lag tactic, really I should have noticed what move he did before this one, and realize that leads to whatever Low/Throw/UB they were setting up.
Take these things offline, and you react better to your opponent. Why?! Because you have been accustomed to reacting to your opponents attacks/movements that much earlier, and that extra training goes a long way when dealing with GOOD offline opponents.
AMIRITE?!