I've put a DD Lite Rail 7.0 on my Sabre Defence 14.5" a few months back and I do notice a difference in overall accuracy from the bench.
Using an ACOG, groups at 100-300yards have been reduced about 20-30% depending on the load. Heavy match ammo performs admirably at long range with the ACOG and DD rail.
The biggest difference I've noticed, as beltfed and TDC pointed out is when shooting from strange positions (urban prone, slings etc) when you sometimes will have stress on the handguards. Shooting from a lot of different positions that put any type of stress will cause a shift in the POI. I have noticed that since going with the DD rail my groups at all ranges have tightened up, most noticablable positional shooting. Most of my shooting is within 125yards using an EOTech for the close stuff.
Its minimal, but enough to notice. I really like how my AR performs with the DD rail, was the best investment in that system. The rifle handles much better overall and I do like the flexible design of a quad rail for whatever I get into in the future.
The rails aren't cheap, I paid just under $400 for mine installed. That's alot of money that could be put to ammo, gear and training before I'd worry about maximizing an AR system for squeezing just a bit more accuracy and no POI issues in my rifle. But....if you have lots of ammo and toys and are looking for a great tweak to an AR, a good quality rail is a good investment, especially if your into tactical stuff where you'll be hanging all kinds of stuff of the front (lights, lasers, IR, vertical grips).
Hope this helps.