My excuse for not doing the same is that it would make balancing the difficulty more complicated. I will have to do some more testing on this.
It might be a good idea to add a possibility to regain some amount of health along the way, I would just have to adjust the difficulty to compensate for this. Randomly appearing rare health power-ups are not an option because that would introduce too much randomness to player's success and final score. Perhaps the enemies could drop items that would give a minimal amount of health to player. But this should only happen in certain conditions..