I'd prefer if invaders didn't heal at all. I don't mind if Host heals if he is actually trying to make it the boss. I like to think of myself as miniboss in the level that the host has to deal with to try and get to the area boss. I will run, hide, ambush, and do whatever I can to try and win, but I think healing with humanity cheapens it.
I like to think of the invader's job is to exhaust the host of as many resources as possible to make going through the level and fighting the boss as hard as possible. Killing him is always the best thing to do, but healing constantly cheapens the whole experience and stifles invader creativity. If a host and his phantoms use a bunch of spells and heal a ton then they are ill equipped to tackle the boss and in a sense, I did my job.
It's also much more fun to play with this mindset, having a war of attrition healing nonstop and getting frustrated because you don't get a kill every invasion is dumb and not the point.