It will shorten your draw length by a minimum of 1/2 inch and can be more, depending upon the size of the loop. Serving wear with a caliper style release aid is minimal. Reserving a bow string is really a small issue, it just requires a serving jig and serving material. Both are readily available online, including examples of serving knots and how to serve a center serving. Bow shops only charge a small nominal fee to reserve a string so it's a matter of how deep you want to get into your gear. I make all my own strings and maintain and service my bows myself. Ol' Sliver Picker even makes his own bows and arrows. It can't be that tricky if a couple of old loggers can manage it,

but I digress.
Whether or not to use a loop basically comes down to a personal choice. I use use a loop on my compounds, but in my judgement it's prolly not worth the hassle, I'm just used to it and I use loop style releases.
It can aid in maintaining peep position, but the truth is, peep tubing will do it better with the exception that peep tubing will eventually break. But, there is no guarantee that the peep position will remain true 100% of the time using the D loop. Even the top commercial after market strings are subject to stretch over time, especially when exposed to overly warm temps and as they stretch the peep will twist.
Best thing that I can suggest is, tie one on, give a try and decide for yourself. I used to think that they were the cat's fanny, but I've mellowed on that...