I think it comes down to where you wheel. If I was climbing boulders all the time, I could see wanting a front locker to walk the front up since it would be difficult to climb straight up a rock without the front. For mud, hills, and what I consider just general off-roading, I'd prefer a rear. In my wheeling I've found more places where I've wanted my rear locker than my front. The extra push from behind has just seemed more useful to me more often and I never use the front unless I'm coming up to a difficult obstacle. If you're doing those difficult obstacles in the first place, I'd have both lockers anyways.
I've rarely found mobility due to the lockers to be a limiting factor on the trail, but I've also never wheeled on trails that are just one continuous obstacle like you see in some places.