I played a game this morning where I decided to try to just focus on holding the XP flag and getting ahead of my opponent in levels. Capture locks were really handy for that, and it worked as far as achieving the goal (fast levelling). On the other hand, though, it was actually a bit of a close thing: there was a point mid-game where he was down 4-5 levels but almost as powerful as me in a fight, thanks to the gear he bought with the money he didn't spend on locks. I'm guessing he was carrying at least one artifact.
On the third hand, though, if I'd had competent team members who would have used the experience advantage well (i.e., not AIs), this might have been a really powerful strategy. I'll have to try it in a game with real people some time.