Right now, when you revive someone, they get half your health. That's a lot of health being given to someone who may very well go do stupid stuff again. Or, of you need to revive several players, you need to decide which one should get half your health, and the other one get a quarter.
I think giving the other player a maximum of 4 health bars (but never over half your health) would encourage people to revive others more. Four bars is enough to survive long enough to get out of danger, collect the hearts on the floor, or pop a few health pills. It isn't enough for most people to feel they can just go charging in again.
This is a good idea. Often, I hesitate to revive others in a tight situation because I believe I can use my 10 health bars better than they can. If it was only 4 or so, I might reconsider leaving them down and out since I'm not giving as much up for them to potentially throw away.