I went around and around with them over this issue. I knew that if they married themselves to MSIE too tightly (it was IE5.5 at the time) it would come back to haunt them. Eventually I quit of the issue. Making IE the corporate browser was one thing, but I truly felt that it was utter foolishness not to do the basic design/development stuff that would allow web applications to work on whatever browser was used.
The argument was always that it is more expensive to support multiple browsers, and I'd argue it isn't any harder to design to W3C specifications.
I wonder how much money ended up being wasted by all the companies out there that adopted the MSIE only philosophy only to then have to spend major money just to test and fix stuff when the next version of MSIE came out.
In the end the inability of companies to leave IE6 behind proves that it is much more cost effective to design to W3C specifications and use open technology as this pretty much future proofs a website/webapp against newer versions of browsers. It is also a smarter way to design from a security standpoint as it doesn't lock an organization to an outdated browser just because their webapps don't work on the newer stuff.