Should Niantic apply a min CP level for pokemons to left guarding a gym?
Most people tend to leave low CP pokemon in gyms just to spite some colors or people. I think it would be better if Niantic applied a min CP level to be able to be left on a gym.
For example: If you are level 20 - 30 you can only assign pokemon above 1500 or more. At that level you are expected to have decent CP pokemon so it would help mantain a gym longer and won't annoy people seeing that low CP pokemon are being left in their gyms.

Answers
Yesterday I fought a 2000 CP Pokemon at the gym and it was more fun than usual. However I don't think that any limits are necessary. There are lots of reasons why people can't put their best Pokemon in gyms, like they need them for raids or are in a hurry. Also don't forget that there are people who just started the game and won't be able to defeat any gym if all the Pokemon there are higher level. And, after all, we put Pokemon there so that they were defeated soon. No need to defend gyms nowadays.
Real defenders might annoy some attackers IF they are fully motivated. But as soon as a guy stops and attacks the gym is lost cuz anyone can win 6 vs 6. Not to mention that motivation makes it even more easier and faster for attackers.
Why Revive the Blissey/Snorlax every time when u can simply throw away the dead Pidgey which collects the same amount of coins in 9/10 cases?