Well, when someone is carrying a single electronic device that does everything, they're going to want to use it on the plane. Besides the Blackberry, you have the likes of the iPhone or Android devices, which are MP3 players, book readers, games platforms, etc. They have Bluetooth, and WiFi, and 3G cell service. Flight Mode is just a way of being sure that all the radio bits are turned off, with a little "plane" logo you can show the flight attendant.
From a bit of searching, it appears that the problem is not really one of RF leakage, but that it's a usability problem. I found
this:
Nevertheless, on a Blackberry, this can be even more confusing because no specific setting titled airplane mode exists. Even without the exact label of Airplane Mode, please note that the option still does exist on your device. By simply going to the main screen on your Blackberry and selecting the “Manage Connections” icon (looks like an antennae), you can then click on the heading that says “Turn All Connections Off” and your phone will be set for use in airplane mode.
How does a flight attendant check that, on every Blackberry model?