I once asked my players in 3e not to add a level of barbarian to every fighter, just because it seems optimal.
I then gave a bunch of trolls one level of barbarian. They agreed it is not funny at all and then we agreed that we stop optimizing without regard for the story of a character. It made the game better.
Is it?
If I read a rule, I always think about how it works if you switch the sides.
Is it funny if you can get 150 temp hp as a player and still cast spells? Probably. Is it funny if your enemy does it? Nope.
So it is about fair play.
Usually I just talk with my players about how I rule. And usually we come to an agreement easily.
Thank you. I do agree.
20 years ago, at the end of 3e, I decided that it is better to stop the arms race between DM and players and it was for the better. Try to play the game together. Don't exploit rules if RAW allow a loophole. If you try, you find a lot of loopholes. The game is so complex that ot is impossible to close each and everyone.