Wasn't it recently changed so that if your friends fall to 0 health, they actually do leave instead of lingering around with 0 health, being able to live for one turn, potentially sponge one hit and then they're done again?
I feel that with that change, it might actually be nice to allow Guest healing in quests. Especially at higher levels, they're likely to be much less capable of taking hits, so it might actually incentivise people to go to heal when they themselves don't need to.
Then again, that does go against the new trend of reducing sponging capabilities, but I don't think it's that big of a deal? Usually if you need help, the fight is likely to be a one-off, so having them at full health might be expected.
Also, it'd make sense with healing pads and such? They work on everyone, right?