As far as I know Yahoo does not delete your emails after 30 days like Microsoft Hotmail does.
If this is the case, this is huge reason to use Yahoo over Microsoft Hotmail because imagine something happens to you for whatever reason, and you can't access your account for 30 days, *poof* ALL YOUR EMAILS disappear if you use Microsoft instead of Yahoo.
Also, you don't get as much if any 'pay to upgrade' Yahoo company spam as you do Microsoft spam, not to mention that Yahoo ads do not cover the entire screen as some Hotmail ads did.
Then there was this thing where people said Microsoft does not delete emails any more. But they do. They send you spam warning you that they will delete your emails if you don't pay to upgrade and a few months ago I tested a hotmail account just to see and sure enough, Microsoft erased all the emails after 30 days so it is not true that Microsoft stopped doing this.
Of course most of us use our email accouts at least once a month but the NOTION that loosing all your emails is always hanging over your head with a Microsoft account is not worth using Microsoft when Yahoo is right there.