Sometimes its useful to have e-mails left on the server. Before I was with Virgin (who do keep e-mails on the server), I lost a whole bunch of e-mails that I wanted as they were to do with my family history and I had to pay out for a program that would retrieve them all. It did retrieve them, plus all the other 100s of e-mails I genuinely wanted deleted!! It's taken me years to go through them all because they just come back with numbers in the title, rather than the original message title.
Now I use Virginmedia, when I accidentally deleted a couple of e-mails a few months back, I just went to the Virgin server, found the e-mails and sent them to myself again. Problem solved. I do delete some of the very old ones on the server sometimes but as, at the moment, there are 8317 on there and they are only using 3% of the MBs involved, it doesn't seem worth it.
Lizzie