Would you powerup a Gyarados with a weak moveset?

It will do what it is designed for - holding a high place without really defending. This is a bit egoistic behaviour, not really strengthen your own teams gyms but taking a high position, but the actual gym-mechanics forces this behaviour.
Just do it, as everybody else does it too.

Of course, you could place a real defender (Steelix or Slowking) at the bottom platform - waiting to be kicked out from your own team (with help of a second account) - you feel good, but this will not be rewarded with the actual system.

Yes, power them up. Because that's what the game requires from players right now. You can feel bad for the lousy gym mechanics, but don't feel bad for yourself in capitalizing on what the game asks of you.

I would love for gym mechanics to be better than this, but as long as it still works like this, I am all for getting high cp Pokemon for better gym placement.

I'm not really sure why there is still so much hate for twister Gyarados after the update. It hits a lot harder than it used to and I simply can't dodge the stupid thing. I haven't faced a crunch Gyarados yet, but of the other charge moves, twister is by far the most annoying. I personally don't see a problem with powering one up and using it in gyms, I would just try to avoid being that person to stick a 3rd or 4th Gyarados in a gym.