I don't see any problem with real cap. I used to run gas guns only, so I'm limited to 30-40 rounds on my rifles.
Now that I run a gaytona with AEG magazines, I still find that I drop my mag and throw them in my dump pouch after 30-40 shots even though the magazines I use are mid-caps (60 rounds).
I also find the users with AEGS using RealCaps or midcaps give Gas Gun users a more competitive play.
Each to their own play-style. You'll have some that want to have a hi-cap and full auto you, and those who like to keep things more realistic.
I also notice that fields that enforce rules about ammo and mag limits they don't enforce it hard enough, what I mean is they never check. You could have a guy with 10 hi-cap mags go on the field and no one bats an eye.