Outlook is pretty much the de facto mail client amongst my clients, even those that aren't in business environments. My largest business client is email-program-agnostic, and still the vast majority of users are asking for Outlook.
Among webmail users, most usage is either Comcast's site, Yahoo's (a result of the SBC partnership), or their internet service provider's.
Actually, I think this might help put it in perspective: more of my clients have email addresses at their own domain versus having Gmail accounts.
A user could, if he so chose, have email addresses at their own domain, on Gmail, AND use Outlook to access it. There's a VERY blurry line between the usage patterns of the average mail user.
That said, I agree with you in thinking the parent was underestimating Outlook's prevalence.
If you include Outlook Express, then you have to consider that it had the same tying advantages that IE had over other browsers. Even so, it's been months since I ran across someone who uses Outlook or Outlook Express for personal email. Thunderbird and Apple Mail are somewhat more common among the people I deal with, but the majority of people I deal with use some kind of web mail for their personal email, with GMail completely dominating among those who are at least slightly technical and among kids.