The only con I know of is they make you adjust your draw length just a little shorter, so you slow your arrow speed down by about 5 FPS. But, you usually gain that speed back. Most people without a loop have 1 or 2 brass nocks, and an eliminator button on the string. You don't need these with a loop (though I use a tied in nock-it weighs basically nothing and doesn't slow your string down at all). The nocks and eliminator buttons can slow your bow down by about 10 FPS or more, so the loop doesn't really hurt speed.

Other than that, you get no nock pinch, inline force behind the arrow, a peep aligner, or basically everything that BTM has in his reply.
I wouldn't be without one, especially not on today's shorter bows.

peashooter