I like having a cover for 3 reasons.
1- the water evaporates much quicker without a cover
2- My p's do slam against the cover when chasing prey and I dont want one landing on the ground.
3- My cat likes to sit on top of the tank and attack my fish. and they like to attack her thru the glass. I would not want to see what would happen if the glass was not there.
You can use a cover or not, I could not care less, these are the reasons I use one.