Think of it this way when you draw your bow your creating a vertical 'V' for a lack of a better term. Most people I know that don't shoot a loop place their release below the nock so this places the arrow on the upper side of the point of this 'V' and actually pulls the string form the throat of the nock and the shorter ATA the bow the worse this is. With the loop you have a drawing point both above and below the nock which creates a flat in the bottom of this 'V' leaving the nock square to the string. As for con's....something else that could break, could be difficult to find in the moment of truth (never been a problem personally, it makes me look down and catch my breath), you may have to shorten the draw length of the bow. The pro's....I think it makes the bow easier to tune, more consistent arrow flight, extends the life of the serving, it also allows you to let down without the arrow falling of the string.
It all comes down to what your comfortable with. People shot bows for a many of year without the loop and it worked just fine and still does today.