The first commandment of mail software is "thou shalt not lose mail". I'm an IMAPper too, but
if POP3 users are losing mail, that has to take priority over everything else.
I'm not sure I accept the idea that Thunderbird needs to increase its userbase, but if we take
that as true, then calendering is definitely the way to do it. If you duplicate the
calendering functionality from Evolution, a million corporate desktops would switch overnight.
Evolution is so flaky I'm starting to think it was developed by creationists out of spite.