Sight glasses are absolutely pointless...I've got a 1 foot lab thermometer that I can hitch up to the top of my kettle...I filled my kettle and marked off what temp marking was what volume. I've never failed to hit my intended post boil volume to withing half a quart. I'd rather have to wipe off a thermo then clean a sight glass, and I would never pay for one. Just my opinion but I have never seen the need for these. Then again I am in indoor brewer so season and humidity play a smaller role in my boils.
Just to reinforce that idea. I did my first recipe this morning with leaf instead of pellet, and my first brew under 8%. Estimated absorption from hops and loss of absorption from less grain and still hit my mark. Just had to delay my 10 minute addition by 3 minute
Its the first page of my brew log. I mark what temp (C and F) equals what volume and then do some simple math to find the exact amount each degree (or five degrees) is per quart. Very simple, takes 10 minutes tops, and you can even be heating up your water while doing it so it actually doesn't take up an extra time.
So you are calling something you don't own and have never used absolutely pointless? Just because you have found a zero cost alternative that you are satisfied with, doesn't make them pointless. They are incredibly useful, and the cleaning process being overly difficult is completely exaggerated.