pic: Swerve!

[NoodleKnight]

The window motor is back drivable, it's just extremely difficult and you'll more than likely break something in the process. Also some of the window motors are known to have a significant amount of back lash (although there was a trick with using keystock to solve this problem).
Also, take into consideration, how much force does it take to back drive the wheels? When will you be encountering this force? The modules are shielded on all sides and if another robot is sticking its manipulator into those modules to try and move them then they're going to encounter a penalty. I'm not saying using the window motors is a bad idea, I've seen teams use them instead of the globes; but same vice-versa, many teams use the globes to steer their swerve modules and have had no problems with back-driving -- since both motors seem to get the job done, I think that whether they are backdrivable or not shouldn't be the determining factor for whether they should be the steering motors or not.