People are so annoyed with spam, that they seem willing to give up usability for a little better protection. Case in point, Yahoo! Mail versus Gmail.
Interestingly, I use both services for certain things, and I have found both pretty good at filtering spam. I’ve also found both about as useable as you can expect from a webmail interface. Nevertheless, I’ll stick to POP, as I don’t really care about the storage capabilities I get from outside services. I’d rather be able to drag and drop an mbox folder to my desktop, zip and encrypt – lock it away for safekeeping, away from any potential prying eyes, but still have ready access myself.
That’s my idea of usability.