Here's how I look at the use of an IBEC on gas powered planes.
If I do have 2 batteries in any plane I fly, those 2 batteries will be in parallel to the receiver through individual switches for redundancy. With 2 batteries where one is dedicated to the receiver/servos and the other ignition, you have no redundancy. You also have a safety issue, where if the receiver pack or switch harness fails open, you WILL crash at whatever the throttle setting was at the moment power was lost. In contrast, in a single batter IBEC setup as I use in my small (15-20cc) models, if the battery fails and power is lost to the receiver, power is also lost to the ignition. A safer situation in my book, keeping in mind that with no power to the receiver no radio failsafe settings will kill the engine.
On my 30cc plane (biggest I fly) I feel that the two A123 packs through 2 switches to the receiver and an IBEC provide an increased level of redundancy without the need for three batteries. As far as the IBEC itself failing, well if it fails open in flight you're gonna be calling out deadstick, and if it fails to turn off the ignition, you'll probably find out when you go to kill the engine after landing, in which case you'll have to use the throttle kill, I do use both
A happy Tech-Areo user!
Pete