IMAP is not a requirement, but it is a real benefit in the specific case where messages are being accessed from more than one device. With a POP account, if your wife's iPhone accesses a message first it will never appear on her MBA and vice versa. This is a royal pain in the nether regions of the anatomy.
If the email provider does not support IMAP my recommended workaround is to get a Google email account — which does support IMAP. Then the Google account can be setup to pull in messages from the POP account — or accounts for that matter. You access the Google account using Mail on the iPhone and MBA but you see the messages from the POP account(s) as if it were an IMAP account. You can even setup the Google account so that if you reply to a message received from the POP account through the Google account it will appear to the recipient as if it came through the POP account.
I hope that is clear. It took me a while to wrap my head around all that, but I have used it for several years now and it works fine. My only complaint is Google's IMAP is a bit different from other IMAP servers in the way it handles subordinate mailboxes.
