I'm of two minds on this issue - I generally advise users NOT to use Word as an email editor, although I use it myself. Back in the bad old days of Office 97, I used to actually mandate that users NOT use Word as an email editor because it was a resource hog and froze up most of the time anyway. With Office XP and 2003, I have found that it usually works pretty well and rarely causes problems. So, these days I give my users the option, although I do think for most users it's not necessary. When we set up new workstations, we change the default settings to use Rich Text and not use Word as an email editor. But we don't prevent anyone from changing it back, if they're sophisticated enough to know how to do it and why.
The reasons I use it myself have to do with formatting issues. Because of the types of information I send to my users, like instructions on how to use various programs (like Word itself, etc.), I often will insert some graphics (a screen capture, for example) into an email message, and I also frequently use formatting such as indented, numbered or bulleted paragraphs. Other formatting functions I use occasionally include tables and multiple fonts. But it's all in the interests of clarity - I use this formatting to make the information easier to read and understand. Using Word as an email editor makes it 1,000 times easier to write my email message without having to use manual formatting or leave Outlook, create a separate Word doc, and then attach it to an email message.