I'd rather look through the center of the glass than the edge of the glass. If you can dial past whatever your max range is without a 20 moa rail, the 20 moa rail is just goin to take you further away from being optically centered at most of your normal ranges.
Basically if you don't need it, it doesn't help at all and possibly hurts. Most scopes have the best picture when centered, this is less noticeable on high end scopes, but it's there.
It depends on the amount of scope adjustment built in to the scope. It is never recommended to run the scopes all the way to the end of it's adjustment range. It is best to keep the scope adjustments Centered in the range needed so it is important to pick the right MOA rail.To much is just as bad as to little.
The adjustments have to have some clearance it the threads to work so in addition to possibly damaging the adjustment screws, the error gets greater as you get farther away from the center.
Chose the minimum distance you will be shooting, then the maximum (Be realistic) and then decide what MOA will keep you in the range without bottoming out the scope adjustments based on drop charts/trajectory.
I dont like rails much prefer 1 piece rings and base like a talley or the new leupold back country
Hawkins precision is making 1 piece rings and bases with a 25 moa cant as well unfortunately not for my rifle at this time but most other bases covered
Many times a 5 or 10 MOA base will get you in the center of the scope adjustment without having to bottom out a scope. With many new scopes and flatter cartridges the need for any MOA added Base is not necessary, especially if not shooting over 7 or 800 yards.
Again, centering the reticle in the middle of the range used is more accurate for the scope adjustments and easier on the scope.
I like the modularity and repeatability of pictinny rails & rings. For me they make the most sense in most cases (my LW-SR build wears one-piece Talley's), but I'll be the first to admit that they are not the nicest looking. Hard to beat traditional bases & rings or one-piece mounts like the Talley's for best esthetics.
Basically, some folks don’t like rails. Not sure if any 1 piece mount ring setups exist but they would be fairly complicated in design. The mount portion would have the elevation built in but the rings would need to be milled with slope so as to accommodate the scope without uneven pressures. I personally like rails with a 20 MOA adjustment for longer range stuff, but for my standard hunting guns I use talley or DNZ single piece mount ring setup. Less moving parts equates to less potential failure
I'm still old school when it comes to mounts and rings. I prefer Leupold 2pc bases and Leupold rings. They look teriffic and I've never had a ring/base failure. I rarely shoot beyond 5-600yds, and then I only do that at the range or when shooting prairie dogs.
As to why not, mounting the scope on a slope changes the look of the gun. That may or may not be a good thing. It also raises the ocular lens and requiring your eye to be further elevated above the stock. Again, may or may not be a good thing. As JE said, ideally the scope would stay optically centered. Depending on what issues may exist with a particular gun a sloped base may be needed to center the scope even at close range.
I think the need for a 20MOA is cartridge/scope/range dependent. I don’t believe there would be any negative effect going with one, other then loosing a bit of clearance between the scopes objective end and the barrel. I have LRH rifles with and without and have never had an issue with my 0 MOA rails making accurate shots to 1200 yards with my 30mm tubed scopes, and my LRH cartridges with velocities in the +2950, and g7 BC’s of +.3.