almost all enterprise companies use exchange. The thing is though that iphone and android have supported exchange for a while, there really isn't much reason to go to the windows phone just to get push email.
Microsoft probably has the most experience with supporting enterprise customers. Apple is largely consumer-oriented and Google is something else entirely. They might match each other on features, but that's not all businesses look for.
Could this be the perfect point for MS to convert existing RIM customers to Windows phones? I understand what you're saying but from a business perspective I was wondering if MS taking over RIM makes any sense at all