When you fly, you are thought to never rely on the fuel gauge. They are notoriously faulty (unless you have a fuel metering device). Instead, you calculate your fuel need before you take off and make sure the fuel is in the tanks before you take off. Its an essential part of preflight to "actually" take the fuel cap off and see the fuel in the tank. It should give you a warm fuzzy feeling (but only if you sniff it). A good pilot can tell you how much fuel he/she has at any point in the flight without a fuel gauge. I think the same can be said for any vehicle you operate.
I don't think a sight glass is a bad idea, you would have to break a few laws of physics for it to give an improper reading. Just saying it might make you lazy.