Corporate: the only reason for this is because web apps and sites are written for a particular browser, and not based on standards.

Awareness: when sites don't work, they'll be aware of the problems. The truth is, most users at getting more educated about browsers all the time. It can't e helped that Microsoft has tied browser innovation and standardisation to their operating system.

Familiarity: really fairly bogus as an argument. There is really not that much different between the interfaces of Opera, Chrome, Internet Explorer and Firefox these days!