That's a shame, really, because Outlook (and Outlook Express) was just so terrible at managing email. I got my parents switched off of that years ago though, so haven't looked at Thunderbird since.
These days nearly everyone uses web mail. Whether it is Zimbra or OWA, the quality of webmail has gotten so good that rarely do you want to do anything else except on your mobile devices. And if you want to go offline, Exchange uses to expect to go to Outlook and Zimbra has its own client too. The number of "working offline on non-mobile devices IMAP users" are dwindling. Thunderbird and Evolution cover that gap pretty well.