Well, speaking from experience, you have a lot to learn.
I've played with this a great deal. I have ARB's both front and rear, and I've hacked the wiring harness so I can lock the front diff independently from the rear.
Bar none, the rear locker helps out the most.
You're right that the front locker will help you get up an obstacle. The front locker is great for that. But typically, the rear tires follow the front tires. So the rear tires would, too, need some traction.
I have found, in almost all circumstances, that once the front of a rig climbs an obstacle, that the weight, or load, transfers to the rear axle. Some would call this "unloading" the front axle. The engine weight is not going to help you here, unless your engine is sitting in front of the front axle. Without a little weight, the front axle will float and traction will be minimized. Of course the front axle will still be getting traction, but now your rear axle is "loaded" - you know this because the rear tires, when aired down, are deflecting. You can see in this pic how the rear tires are deforming under load.
Of course in this picture the front axle is unlocked. On this obstacle, you need both traction and turning ability. With the front axle locked, turning the truck is bit of a trick. With the front axle locked, the turning radius is so wide, maneuvering through the "trail" would be rough.
Here is another example. On this obstacle, as you can see, a rear locker is needed. In fact, I did this obstacle numerous times locked, unlocked, front only, rear only, etc... I was able to do this obstacle with the rear only locked, but with only the front locked, I was unable to pull through. Really, I can't understand why:
But again, it's easy to see how the front axle "unloads", thus little traction is provided.
Here is a picture of traction, just incase someone missed it:
Sometimes you want the wheels with traction to actually turn. Sometimes.
Now, with an open diff in the rear, the energy, or force to propel you forward (or backwards) will take the path of least resistance. So if one rear tire is getting awesome traction (see above) it's the other tire that's going to spin.
Of course, sometime you need traction going down a hill. Or what if you're needing to back-up a hill? You may need a traction aid there, too. IN these cases, a front only locker may be more help.
I spend more time driving forward. Maybe some people spend more time driving backwards, though. I'm not sure. I have never really witnessed this, but maybe I need more experience, or travel to other parts of the World to see this.
There is always some wise-one who say's "well, I don't do rocks like that" or they will comment on the condition of my beater truck. So, here is another pic of my girlfriend in her LR3. As you can see, the front-end is unloaded...a front locker will not help nearly as much as a rear locker here:
It's not hard to understand that the rear tires need to climb here. The front end is light.
Now if we were backing up this obstacle, the front locker would be really nice to have.
Again, front locker not so good here (if you can only have one):
Ok, one more: