As you know, the AR15 was originally designed as a 20" rifle. The timing, parts, etc. were all designed with this in mind.
Now when you cut the barrel down and move the gas port back, two things happen - one is that the gas reaches the gas port much faster, the second is that the gas pressure is much higher (the same amount of gas from combustion of the cartridge is contained in a smaller space). For example, the port pressure of a rifle can be from 12-15k psi depending on how eroded the gas port is. On a carbine, that port pressure is more than doubled to 23-30k psi.
A higher port pressure that reaches the bolt carrier twice as fast (shorter distance to cover) means that the rifle tries to unlock earlier and unlocks more violently when it does. Most of what we now consider "standard" M4 modifications are designed to address this issue: for example, heavy buffers slow the unlocking by adding more weight for the bolt carrier to push against. Heavy-duty magazine springs push rounds into position faster so that they can be in place when the bolt comes back slightly faster than the rifle. The extractor inserts, O-rings and spring all provide extra tension on the extractor so that it can hold on to the case as it tries to rip it out of the chamber earlier than a rifle would.
Another issue that carbines see is micro-cracking of the bolt that can develop as soon as 3k rounds and has almost always developed by 6k rounds. One reason for this is that by increasing the port pressures, you also increase the pressures on the bolt lugs by as much as 150%. This is one reason there was a spate of "improved" bolt designs for some time. The problem though is that it is hard to improve the bolt without redesigning the barrel extension as well and it creates maintenance and logistics issues.
The midlengths move the gas port out another two inches to more closely match the operating pressures and cycle times of the rifle. Now keep in mind, the difference in unlock time for a rifle and a carbine is 175 microseconds (a microsecond is 1/millionth of a second). The difference between a midlength and a carbine is even smaller. So we aren't talking about dramatic, night and day improvements here. A reliable carbine from LMT or Colt might outperform a Model 1 midlength in reliability and longevity. There are just too many other factors involved.
However, the main reason people like the midlengths is they give you extra sight radius and a slightly more "original" gas pressure and unlock time. Over time, that should translate into less wear and tear on parts as well; but as far as I know nobody has actually done a study to verify that. They also recoil slightly smoother (my subjective feelings) compared to the carbine.
