I have set up a few bows where the folks in question asked to have a string loop set up with a brass nockset. It seems to work well but that particular combination also adds alot of weight to the center of the bowstring and thus reduces arrow speed more than just a loop or just a nockset/e-button combo.

As for the loop, well, there was a post on here a week or two back where someone(I apologize for forgetting who) posted a very strong case "against" the use of a string loop. It went through each of the major "selling points" of the string loop and offered a very concise rebuttal.

Personally, I switched last year and have noticed a slight increase in accuracy with them. I am assuming this is because of the decreased amount of nock pinch (30 inch draw on a 33 inch long bow) and also because the force is more evenly distributed behind the arrow nock.

As most folks mentioned though, nock pinch and serving wear are the two primary reasons that are used to recommend a loop.