I also had a Blackberry for a couple of years and used multiple Pop accounts without one hitch. Your explanation as to why this is the case makes complete sense to me, but what doesn't is the bottom line; are you saying Blackberry is ultimately superior when you want to use Pop accounts with multiple machines?
I'm saying RIM provides a dedicated email delivery service for the Blackberry which is doing all the incoming mail server checking. Not sure how this is different from an email client checking the incoming mail server for an account but it must be. And when RIM's servers go down (which happens from time to time), no email from any account.
Although it is possible to access a POP account with more than one email client, a POP account is designed to be accessed with a single email client only. If you are accessing a POP account with multiple email clients, you need to use only one email client at a time that is actively checking the incoming mail server for the account for new messages.
This is not a problem with an IMAP account, which is designed to be accessed with multiple email clients at the same time with the ability to have multiple email clients actively checking server stored mailboxes for the account at the same time.