Exchange is the de-facto standard for mail in companies with 1000+ employees (probably smaller companies as well), and compared to IMAP offers
* push email
* contacts (and calendar) sync via the same account
* enforcing corporate policies such as a requiring a device password and a maximum lock timeout.
Support for Exchange on Android and iPhone has provided a huge boost to the popularity of BYOD and is one of the main reasons why Blackberry is now failing. Previously companies had to buy RIM servers and devices and give them out to employees needing email access on the go. Now only an Exchange server with Exchange Activesync support is requred and any phone can be used, even cheap chinese phones like ZTE and Huawei where previously >$700 Blackberries were required. Most people also prefer to carry just one phone for their personal and corporate use, and for company-issued phones Android phones, even with the MS tax, have a much more attractive price than iPhone. So adding Exchange Activesync is probably the cheapest and easiest solution for companies already using Exchange, and adding support for it in all Android phones is a smart move by Google - together with Apple and Microsoft they practically stole Blackberry's customers.
I'm sorry, but the only alternatives to Exchange are:
* hosted email on Gmail, Yahoo etc. - not good for security/privacy considering the recent hacking of LinkedIn, Adobe and many other companies
* using a set of open-source products which would provide a scalable solution for intergated email, calendar and contacts; a desktop and web-based interface; and support for major mobile platforms.
* Lotus Notes. If you want users to kill themselves out of frustration