For a single user on a home system picking up mail from an ISP's POP mailbox (or three) in the normal way I use Outlook Express largely because it has a newsreader integrated and Outlook never used to have (I don't know about more modern versions).
Also, but this may be a feature of the site where I'm using it, Outlook appears slower to do things like download more than a couple of messages from a POP3 mailbox than Outlook Express, which can download several messages per second on a good day.
On the other hand Outlook has the calendar and Outlook Express doesn't. So it's a balance between newsreader, speed and calendar.