Well, in my case I have a small number of accounts, most of which I started just because the domain name sounded neat or I liked some feature. Or I might have started one to be used for family/friends, another one for sign-ups (message boards, etc.), another for online shopping, another strictly for back-up, etc. But sometimes I change which account I use for what. Then also every so often I prune my list of accounts down to just a few. Call it Spring cleaning. This chaotic behavior is a direct result of my emailophilia.
As things stand now, I have accounts at five providers: Hushmail, MyOpera, Yandex, EuMX and Runbox.
I have Yandex just for fun and to find out what it's like. Multiple accounts that I have at Hushmail are for back-up of important emails. EuMX is a free account offered to EMD members, which I use mainly for general back-up. MyOpera was my main account (nice, blazingly fast interface !), but I have moved my main email to Runbox. It's a pay-only service, but I got two years of the lowest level at half price ! I frankly prefer the peace of mind that I have in knowing that Runbox: 1) is very privacy conscious, 2) is located in Norway with strong privacy laws, and 3) has a very responsive customer support team, who address problems very quickly and courteously — unlike Gmail, Yahoo, etc. where you might have to wait for a very long time before getting customer support (if you get any at all). And in case my account were to be hacked, since they have my CC on file (and IP address, etc.), I'm sure it would not be difficult to get my account back — again, unlike the nightmare of account recovery one must go through with Yahoo, Gmail, Hotmail, etc.