Form follows function, and the need for an adjustable cheek piece is determined by how high your cheek must rise above the stock to get a full sight picture. The adjustable cheek piece thus provides a repeatable cheek-weld. The cheek piece needs to be adjustable not only if the shooter switches between glass and irons, but also in the case of target boltguns, to facilitate the removal of the bolt. If the cheekpiece was simply built up, you would have to remove the action from the stock to withdraw the bolt.
From this need we come to a diversion as how to best answer the problem. The most traditional and the cheapest method is some sort of saddle arrangement that is attached to the outside of the stock. Early solutions were leather pads laced to the stocks of conventionally stocked rifles. This has evolved to plastic, fiberglass, and nylon saddles. It is also the worst way top solve the problem.
All of the saddle style cheek pieces offset the face from the centerline of the stock and this builds in a windage error. This error can be corrected for, and might not be a big deal for a non-precision rifle. But if you are going to invest the money into a precision rifle, I don't know why you would want to build error into your system. A cheek piece that has internal adjustments provides the correct height without a built in error.
Consider the following test. Fire 5 rounds at a 100 yard target with a plain stocked rifle. Now place a thin leather glove between your cheek and the stock and fire another 5 rounds making no adjustment to the sight. Do you think the 2 groups will have the same POI?
