Not to mention that if your software sends out emails with links to your application, chances are high that your client's email client (Outlook) will open the links in the default browser (which still might be IE).
So even if you can convince them to use a different browser, be prepared to tell them how to configure the browser to be the default one.
That's interesting. Part of why we started supporting IE is because some of our most important users somehow ended up on our website in IE, despite not normally using IE. Wonder if this is the only reason why.
So even if you can convince them to use a different browser, be prepared to tell them how to configure the browser to be the default one.