Whether balance is important in a single-player game or not, my personal opinion here is this:
A talent that is making you practically immortal, in a game where challenge is definitely supposed to be part of the fun (challenge to some degree, of course, learning some useful tactics will help a lot), is unlikely to be the way the devs intended it to be.
Sure, you can not take it and avoid the feeling of cheating the game that way, but that's not really satisfying, imho.
After all the concept of leech is interesting. It's the execution that might need some tweaking.
