In flight mode with the screen off the battery drain is similar to other phones that "switch off" but still wake you up with their alarms. Microsoft's method, it could be argued, is better than the way other phones operate. When you switch your phone off it really is off, and won't wake up until you physically switch it on again.
In your own terms think of a business user who regularly goes on long haul flights, with your D900, e61 etc. you get on a plane, switch your phone off and put in into your pocket/bag. During the flight alarm time comes around and the phone switches on to sound the alarm. At the same time it also switches the radio back on, so not only do you have to contend with the embarrassment of getting your phone out to switch the alarm off, it could also interfere with the plane (if airlines are to be believed). With a MS Smartphone you can switch it off getting onto the plane and not have to worry about it coming back on until you tell it to, whether your flight is 1 hour long or 16 hours. Which do you think is more business user friendly?