I don't know about you, but when I'm at home I use an e-mailclient to read/sent my e-mails. However, when I'm not at home I use webmail clients and in that case I don't want to save any personal information so other, unauthorized, people will be able to read my e-mail as well.
But that's just my opinion...
Maybe you use webmail for another reason, but remember that saving passwords is always a risk!