Yahoo will go the way of Excite, Lycos and Alta Vista (I actually really liked their search pre Google) very soon. Their email system completely ignores any filters you add with regards to spam which is all I ever get from it, to the point that I no longer check it. The only thing I've used Yahoo for in the past 10 years is fantasy sports garbage which I'm not at all married to and can just go to another provider, no problemo.
I never use Bing as much as I probably should because Microsoft's Hotmail has always been rock solid and scalable all the while being a free email service I've used for many many years. My Hotmail account is trained like a good watchdog which is more than I can say for Yahoo's system of blocking.