Most motors come with a single ended shaft that can be set up for front or rear mount.  If you front mount the motor and want to add a rear support bearing a bolt on prop adaptor is used.  This adds length to the set up.  On a profile ship noise length is at a premium unless you bury the battery in the leading edge of the wing (could be messy).

So the question is if a really rigid front mount is used how necessary is the rear bearing support?   Also does anyone make after market double ended shafts to replace a single ended shafts?

Hey John:

It's going to vary from motor to motor, but on my eFlight 15, the shaft is plain with no set-screw flats or circlip grooves or anything.  Given a shaft like that, all you should need to do is get a precision ground shaft in the right diameter (from someplace like McMaster-Carr), cut it off to length, and put it in the motor.

The two potential problems that I can see to that are that the motor manufacturer used a slightly over- or undersized shaft, or that you have a motor where they got fancy and used a circlip (in which case you need to sweet-talk a machinist into making you a shaft with a groove).  I doubt that either is likely, and a flat is something that you could probably cope with.