Matter of "Style," I guess
I learned to shoot the .45 auto with an arched MSH, during a time when there was a lot of emphasis on one-handed shooting, both from waist level and more-or-less unsighted fire from shoulder level. By the time I tried to use a flat MSH, I probably had a lot of muscle memory working, and I seemed to hit 'way low with it.
I also read of the reasoning behind the switch to arched MSH on the 1911, and this seemed to tally with my experience. As soon as I could afford a Colt National Match (The early "Gold Cup") I got one. As it was conceived as a bullseye patch pistol, it came with a flat MSH. I started switching arched for flat about that time, and have been doing it ever since. Maybe this is the reason I like the old Commanders and Govt Models so much--They came with arched.
Really, I can make do with either one, but prefer arched. In that so many people were making bulleye guns do duty for fighting guns, it became popular to install flat MSHs, first by the gun smiths, and later by the various factories. I believe it is mostly a fad, like "Full Length" guide rods. A matter of personal preference, mostly.
Best,
Johnny