A selectable on the front, will allow easier steering when you don't need to be locked up. If your front diff is locked, it puts alot of added strain on your steering parts, to the point that steering is really tough, and the only way to combat that, would be to unlock one hub. Or go with Hydro assist steering.
Here is a basic layout of a turning radius.
Basically, look at the lines in a basic left and right line, in a right hand turn with your front diff in a "open" position, the right tire will go a shorter distance (rotates less) than the left tire (which rotates more). The tighter the turn, the greater the amount of travel difference between the tires. Now, if your diff is locked, both tires will want to go at the same rate of speed. When you try to turn, the axle will fight you and want to keep going straight. Why? Because both tires are locked together, and will want to travel at the same speed, wanting to go straight. Now, when you force the steering wheel to turn the tires, your putting GREAT amount of pressure on your steering box mount, linkages, TRE's and steering arms. And then there's the force through your axles trying to drive your wheels. Because both tires want to rotate at the same speed, but your forcing them to rotate at different speeds, now the added torque is put on the diff, axles, birf, and hubs.
Where as fully locked in the rear, will put a load on diff, axle shafts, and tires. Generally, the tires are the one that takes the brunt of it, by wearing out faster. I'd rather have my rear tires wear out faster, than have my entire front end get hammered, and have to worry about breaking off steering arm, cracking the frame, or blowing a birf, just on a simple drive.
Hopefully this helps..
