One of the fun things about the AR-15 / M4 style rifles is that they are easy to mix/match with a huge variety of accessories, including optics. That same flexibility can also be confusing. For example, a short barrel rifle can shoot very accurately at long distances, so why not use a high power scope? My answer to that question is, “Go ahead.” However, for my rifles, I use the standard of, “Have a rationale.”
For example, pictured above is The Trijicon MRO on the Colt Trooper. I try to use a less-expensive optic with a less-expensive rifle. Both of these items are relatively inexpensive compared to their competitors. I don't mind (as much) if this combination gets beat-up, dropped, or stolen. Additionally, this rifle also draws fewer looks at the range, or elsewhere, because there is nothing unusual about it.
Another rationale is to have a rifle that is easier to keep hidden, and easier to use in confined spaces. Pictured below is small optic on a small rifle ( a BCM 11.5 inch Short Barrel Rifle (SBR) with an Aimpoint Micro H-1).
Another rifle with a rationale is a “Versatile rifle.” Pictured below is a 14.5-inch SBR with a Trijicon Accupower 1-4 variable power scope. This rifle is attempting to achieve the ‘Goldilocks’ standard.
Any of these rifles have certain attributes that aren’t perfect for all tasks. However, each rifle demonstrates a rational combination of the many optics and accessories available to us.