I prefer glass tops because of issues like debris and moisture.
If your lights are exposed to an open top tank, it's best to make sure moisture can't penetrate anything important: Moisture resistant end-caps, no exposed wires under wire nuts, etc.
Even if you do take precautions, equipment could potentially wear down quicker, or IMO be more troublesome. The light housing, reflectors, and lamps are subject to those irritating hard water deposits that end up on your glass. I'd rather wipe a glass top down over a whole light fixture, and I'd rather not pay for more expensive parts like moisture resistant endcaps; at least not until I become filthy rich and use softer water.
A glass top definitely helps protect you equipment. I don't know how not having one works so well for some.
I'll probably try something like the sliding glass option for my current canopy project.