Blackberries on BES offer enterprise features simply unheard of with Winmobile or iphone devices. Windows mobile only recently got the much needed security features such as remote device deactivation and wiping. Blackberries simply offer more for the enterprise such as a slew of custom encryption features, mds for intranet based apps, web proxy features so you can control user's network browsing, full featured logging (down to the phone calls you make) fully customizable IT and security policies, and I'm sure I'm missing a ton of other features that Blackberry offers that has not even been contemplated for Winmobile much less iphones.
Maybe you should be offering your customers reasons to use Blackberries and not reasons to make your job easier. You never know, they may think that one of those features you don't care too much about is pretty nifty...