I think that there is just not much for most players on a virtual bomber to do from one minute to the next. Take your example of a player in one of the waist gunner positions. What does he do for the take off? During joining the formation? Over the channel? In the skies over Germany when there are no fighters? (observe the sky is the answer to that, of course, but I can imagine that being rather boring after a few minutes of nothing happening.) What about during bomb runs, or flying over flak concentrations?
Also, who decides when to use time acceleration so that the bomber (or bombers) can bypass the boring bits of the flight? What happens if and when a player dies early in a mission? Do they rejoin on another bomber?
You can try out the idea of taking a single role on an aircraft by playing this...
https://www.gog.com/game/b_17_flying_fortress_the_mighty_8th...and seeing how much fun it actually is. On the whole, I suspect that being able to switch roles makes for a more satisfying game experience. Of course, there is the social aspect, but I imagine that players will get a very uneven experience based on the role they take on the crew.
On the other hand, emerging VR technologies might make the whole thing a more satisfying experience than would have been possible with a game like The Mighty 8th. Being able to clamber around a virtual bomber, doing things like manning guns, giving first aid, repairing battle damage, and bailing out etc. etc. might be more fun in a well executed virtual environment, provided that a sensible approach to time acceleration is taken.