Litmus has a very large survey that has estimated Outlook's market share at 43%, iPhone clients at 4%, Apple Mail at 4% and Thunderbird at 2%. Of course that includes Outlook Express in the Outlook figures but I don't know if that works for your situation either. Campaign Monitor has a different and more recent view indicating a much larger 16% iOS client users and 11% Apple Mail users, and a smaller 23% Outlook users.
They must be using different metrics as I doubt Apple Mail and iOS mail grew quite that much in the last year, but I think the general picture shows that a significant amount of your potential users may be using such an email client. Of course it depends on your specific audience, but I wouldn't want to alienate anywhere near such a significant portion of users.
As Roger Attril mentioned the Campaign Monitor site's numbers only detect email clients that open images; here's their fine print:
The email client a person is using can only be detected if images are displayed. This can give an inflated weighting to email clients that display images by default, such as Outlook 2000 and the iPhone. It will also provide a lesser weighting to those that block images by default such as Gmail and Outlook 2007. Those email clients that aren't capable of displaying images, such as older Blackberry models and other mobile devices cannot be included in this study.