I wouldn't expect to have professional development overtly included in a short term contract, either. I'd expect a much higher hourly rate to pay for it though, amongst other things.
I'd expect if the plumber worked for a plumbing company they'd probably invest some time and money into skills growth, and pay for it out of the money they charge you.
I would like a competent plumber and if he does some fancy work in his spare time and gets even better than great.
But I would raise an eyebrow if beside my work he will also want to be paid for 6 hours he spent in the weekend, experimenting on his bathroom...