"- does Yahoo have free IMAP or POP? No"

Hmm... then how does the iPhone client fetch mail from Yahoo?

"- does Yahoo have a decent SPAM filter? No"

I get more spam in my INBOX in my GMail account (which I have protected quite steadfastly) than my Yahoo account.

" - how much free space for storage? Too little "

Are you kidding me? Yahoo went to "unlimited" storage like 5 years ago!

Looks like you have a pretty strong bias. Time to re-think those assumptions...