To be honest, I never thought further ahead than how I would feel looking at a wing covered in ice (and only marginally less ice if I used warm water), that minutes earlier had only been covered in frost. And how would I get the ice off.
If we are doing the guessing game:
It could be hard to dry the aircraft, and in cold conditions, the water wouldn't just evaporate, it would probably go somewhere you didn't want it to, and then freeze, possibly locking a control. Or contaminate a surface. The weight increase would not be an issue, but the lift/drag changes could be.