I don't mind the concept of paid add-ons (for flyables). It also means the core game can be released earlier and the ongoing development of new content assures the game will be supported for a long time (bug fixes, upgrades etc.).
I think BoB:SoW will use a similar concept but Oleg chose to give us more aircraft to fly with in the initial release, probably one of the reasons it takes a little more time to release.
I hope the concept wil work. I remember a similar appraoch was to be used for a racing game called "Racing Legends" by the West brothers some years ago. They were going to release racing cars as seperate addons to the core game; but each release would (could) be published as a seperate DVD with (paper) manual, original photos etc. They even planned for "collector item" or deluxe versions in leather casing etc
It was a great idea, but unfortenately they never delivered.
Update: I found that the link is still active!
http://www.racing-legends.com/expansion_packs.htm