For a short while I worked for close to minimum wage washing dishes in restaurants. On my breaks I would read microprocessor manuals. One day, a customer saw me

For a short while I worked for close to minimum wage washing dishes in restaurants. On my breaks I would read microprocessor manuals. One day, a customer saw me reading such a book and we struck up a conversation. He worked at a small technical shop where everyone was basically commission-only for repairing computers. I knew quite a bit about electronics and we were one of the few shops in the area that could repair monitors. (This was back when they were expensive.) To make a long story short, today I'm a Senior Software Developer at a large company. I'm happy enough without any aspirations to management at this time :-) I agree with you that you don't learn a lot of great marketable skills by scrubbing things clean. But such a job, by itself, can't stop you from learning if you're determined.