I don't disagree that it would promote "smart play". It certainly would.
So would following the PnP rules of decreasing someone by a level every time they died. Now there's a challenge for you.
What I'm saying is that it would require far far more out of your casters than your meleeists, without the additional benefit to the Caster.
Casters are currently balanced based on the premise that the Caster CAN stand in that firewall (which is a tactic, incidentally). Life for the Meleeist on the other hand would change relatively little.
See the imbalance?
In order to get the same effect the Caster must dramatically alter their playstyle. The Meleeist not so much. The caster must do all this for arguably less benefit than they provide now (They can spam fireballs at villains built for that... a single fireball isn't going to mean squat).
So the Caster either becomes buffbot to the meelee, or well... nothing else.
They can't cast web, hypno, and can only manage with single target controls.
So their Offensive spells are neutered, their defensive spells are neutered, leaving them with single target attacks.
Why not just pick up another fighter?