Loops do not affect drawlength. They affect anchor. Your correct drawlength is your correct drawlength, regardless of what style you shoot--fingers, short release, long release, under nock, loop, whatever. If you lengthen your drawlength by an inch to " get it back to your anchor point" after removing a loop, then you are running too long of a drawlength and accuracy will suffer because you will be stretched out and everything is not in correct alignment for the best repeatable accuracy.
I shot off the string for many years when I shot fingers and then first started shooting release. Then I went to a rope release which allowed better shooting characteristics because there was less torque imparted onto the string. I had to change my anchor because of it, and I had to adjust my anchor, but it quickly came around. I changed to a loop about 10 years ago, and have never looked back. They have many attributes once set-up properly and are much more forgiving. I set mine up with a tied-in nockset both above and below the arrow nock (not too tight, you want to give the arrow a bit of space for full draw, at least 1/16 at rest between bottom of arrow nock and bottom nockset will do) Then I throw the loop above and under them. This sets up the loop to be in line with the arrow, and will not " cinch" the arrow like a straight loop will do and cause erratic arrow flight. If you ever happen to pull through one in the field, with the two nocksets install you have quick reference points and can change one out in less than a minute. No wear on serving from release jaws, string maintenence is minimal. No torque, no eliminator buttons, no rubber-tube peep. (You can use a tru-peep and it will come around perfectly with little diddling. This picks up about 15 fps just by eliminating the tube) yes, there are reasons why most archers shoot a loop now.
I set-up archers with both loops and no-loops. It is one of those things that is a personal preference issue. But I can tell you that the loop does have it' s place, and when setup correctly works extremely well.

Good shooting, Pinwheel 12