IIS is not required, WDM uses Tomcat that gets installed as part of the BAS installation.
However, I did leverage IIS to create a redirector so I could give users a friendly http:blackberry.mydomain.com URL to use rather than having them need to remember httpS:myblackberryservername/webdesktop. Maybe you could accomplish the redirector with Tomcat, I don't know. But since it was dead simple to do it in IIS I just took the path of least resistance
(Home Directory tab of the default website properties, set content to redirect to a URL rather than the default local content folder)