I keep an exact round count of all my guns. I have no real good logical reason to, except that I'm OCD about it and for my own personal curiosity. Do you need to? No. But for me, I like numbers, and I like to see how long things last. If I were to ballpark it, I would probably be off by quite a bit. It also helps me to keep track of failures and malfunctions. It can help me see if something is a pattern, or if it's just a one off problem.
As for it being foolishness, I think that's a pretty arrogant platform to stand on. Many things that people do every day are foolishness. Keeping a round count takes literally 20 seconds per gun each time I go out. To say there's not enough time for "such foolishness" is just, well, foolish. If you can't spare 20 seconds (or even a minute or two) then I pity you. I'm not saying you have to keep track, but it's certainly not a burden of time, and most definitely not foolish.