"is learning just for the sake of learning worth it?"
We are called Homo Sapiens, which means Thinking Man, which means we have a reputation to think of, a name to live up to, so... Yes. 
"Haven't you ever taken a say class that was useless at the time and now years later it seems useful? Or conversely now still is utterly useless?"
Commercial Horticulture NVQ Level 1, Amenity Horticulture Level 2... Thjat means I can run a garden centre/shop, and I can do the stuff councils do; maintain public grounds, grass verges, etc...
Totally useless.
Although being good with hydroponics means I'll never want for money 
"So, it's defeat for you, is it? Someday I must meet a similar fate..."