As for the actual topic...
I think it's important that you encounter Kai Leng multiple times, to build him up in the game as a credible threat. Most people haven't gotten into the Mass Effect "EU" so it would be like just throwing someone out there and saying "Ooooh! You should be afraid!" even though you don't know why you should be.
There can be ways to encounter him, without actually getting a fight. We can be put in situations where we're helpless to watch him kill others, or even an encounter (or situation) where he beats us. Maybe not in a stand up fight, nothing like scripting it so we can't win, but through guile, like a trap he sets for Shepard.
Having him kill known NPCs, like a former companion, or cause trouble for others in the game - like destroying a major location, without us actually encountering him personally in a situation where we can fight/stop him, would be another way for him to act and build up his threat level.
All of these allow us to have a "face" antagonist, like Saren of ME1, throughout the game, which is often nice. It also makes it a lot more rewarding when we actually fight/beat Kai Leng. It then MEANS something to us, within the game.
As for the possibility of him surviving ME3, I think that's a fine possibility. It's not that we're "unable" to kill him, as if he's too strong, but simply that we don't. Maybe we think we do, maybe we win the gunfight, but we're busy with other things, don't realize his heart's still beating, and he crawls away. Maybe we just never get the opportunity to finish him off. Etc.