I'm pretty sure I've covered it several times, even on this forum. haha
But I'll try to give you the short version.
Imagine you install a 2" spacer up front. Since the spacer bolts on top of the coil assembly, the suspension will droop an additional 2" beyond stock design. This creates excessive angles on the ball joints and the CV joints. Not only that, but the coils will contact the upper control arm and make noise.
The Bilstein 5100s on the other hand are only a little longer than factory shocks. They will stop the droop before anything is put in a bind. You will only end up with a little over 2" of droop from ride height with the shocks at 2" but the ride is fine. The height is achieved by moving the spring seat for 2" of height, and in the end you actually have more available wheel travel due to a longer shock.
So to recap, 5100s = no binding, no excessive angles, more wheel travel and better ride all around. Not as good as a real lift but much better than spacers.