I barely get any spam at all on FastMail. The spam I do get is classified fairly well, but I haven't even gotten enough spam in the 18 months since I switched to FastMail to get personalized filtering enabled! (200 spam messages)

Assuming spam filtering is "good enough", which it is, I'd rather not have my mail used for helping other people's mail get filtered better. FastMail solely uses email you confirm as spam to help their spam reporting efforts, and you can even disable that setting. No accidental spam gets mined for any purpose outside your own account's benefit.

I also feel Gmail trusts it's own opinion more than yours, with regards to training behavior. I used to have receipts from some stores always marked as "promotions" or even "spam", no matter how many times I tried to teach Gmail otherwise.