Always the contrarian, I find the sight glass on my boil kettle very useful - but I did have to re-calibrate it, as the factory markings were off by over half a gallon. It definitely is worthless during the boil as for whatever reason the wort boils in the glass long before it does in the kettle itself. Without etched markings in the kettle, it is the only way I have any idea how much sweet wort I am collecting from the mash cooler, unless I was to drain the cooler into a measuring vessel and pour from that vessel into the kettle, which just increases the HSA. At this stage of the learning curve, I'm not real anal about HSA, but I certainly try to avoid it where it can be easily done.
A lesson I had to learn and re-learn the hard way over the years, too many times to count - find a way to be able to afford what you want and need and don't buy until you can do so, cuz nothing hurts worse when you are short on funds than wasting what money you do have on something that turns out to be less than what you need or expect. Cheap tools are no bargain.