I disagree, only the other month I was able to identify and call-out a company that had managed to acquire an address which I only gave out to one company.
Turns out, this company director used to work for the company who I'd given the email to. Cue one threatening email (knowing the DPA inside out sometimes comes in handy) and several extremely prompt replies from them, and they backed down (and lost an AWFUL lot of potential custom just by doing something extremely stupid like pinching their former employer's email database).
But, to be honest, that's nothing to do with Yahoo so much as having your own domain, a catch-all email account and the brains to put different email addresses into every website and keep a record of what one you used with who. If you want throwaway email addresses, just buy a domain for a few quid a year - you can use it for EVERYTHING, it doesn't matter what ISP you use or whether they change over to Yahoo, you can have all mail forwarded to any account you like (so if you WANT to use GMail, etc.), and you can do useful things like sticking a website on it, giving friends and family a free, extra email (e.g. email@example.com which just forwards to their email or gives them an IMAP account).
The amount of spam I get is close to zero. Hell, I get THOUSANDS of attempts, but the number that actually pass through is next to zero. Basically, because you have to know what addresses I'm currently using and only the places with valid emails that I've written have that. And when I spot spam, I complain to the only company that was ever given that address and, if necessary, block it. You only really get spam when your addresses are PUBLIC KNOWLEDGE, like putting them on websites, in CV's, etc. or when someone at a company decides to steal/sell the email database. The latter is surprisingly common, I've found, and the former are easily avoided.