I'm a little worried about that rule as well.

On one hand, I can see a story reason why this rule is there: Other than Azzen, we don't exactly have a true healer with the medical knowledge to prevent the exhaustion after bringing someone back from the brink of death. 

However, I can see this rule becoming problematic in Boss battles, and in situations where running away isn't an option. It's also a good reason for party members to abandon each other when things get tough. 

It should also be mentioned that after receiving 6 levels of exhaustion, the result is death. So, if someone was knocked down and healed back six times, they would just die... :/
(Sorry if I'm being Captain Obvious there...>.<)

It also raises this question: If we get to higher levels and have access to Revivify and Resurrection spells, then does that mean that the person who was brought back gets more levels of exhaustion then just the normal healing? 

That's just what I think.