1. What is the benifit to the rear lug recievers?  2. Were these ever used in military rifles?  3.Are they worth the extra cost?

1. What is the benifit to the rear lug recievers?

A rear lugged receiver is to make your bedding life last longer, they really don't improve your accuracy (unless you consider it in the long run with the extended bedding life).

2. Were these ever used in military rifles?

I'm unsure of the Army, but I know the Marines did (and someone correct me if I'm wrong) by welding the rear lugs to a regular M14 receiver for there team rifles.

3.Are they worth the extra cost?

According to George Gardner and GnySgt. Andy Conrad (USMC Ret'd) of G.A. Precision, they said that with the modern bedding compounds available today-I would be better served by saving my money and going with a standard receiver. Gunny Conrad built my USMC DMR clone and he said to rebed when I replace the barrel in about 5000rnds. Gunny Conrad was the NCOIC of the USMC DMR program, so I think he knows a thing or 2 about building accurate M14's .
My DMR is a consistent 1/2-3/4MOA shooter and easily passes the acceptance standard of the USMC which is 10 shots in a 3"x3" square at 300yds. I've got about 2500rnds through it so far with no signs of the accuracy letting up.