Our major product at work is proprietary and completely written for IE only ... I think this is WRONG!
It is a WEB application, thin client, one of the biggest advantages is the the ability to let the client choose which program (browser) he uses to access the application, even if it is installed on a local intranet. I love the freedom to choose the programs I want on my machine.
Since I learned to use standards, and not the "flexible way" of M$, I write my apps for all standards compliant browsers with less efforts than writing it for IE only ...