I agree its more realistic, but only if all players are restricted. Fighting a player with a high cap using a low cap is not realistic and puts the low cap player at a definite disadvantage.
Also, a hit from a 5.56mm bullet tends to put the player out of the game, where as a hit from a single 6mm BB is often ignored, either deliberately or because its minimal impact is not felt through clothing.
In this way a 30 bullet mag of 5.56 could take thirty people out where as 30 BBs might only take a few players out.
I find that many players need a few hits to make them put their hand up at long range and the drop off of BBs even with hop up make the Airsoft gun less effective/accurate.
Of course, one could argue that winding a high cap is more realistic in that it simulates the time it takes to reload and puts the player in harms way, unprotected, in the same way.
Its nice to have the option and I will be getting some low or mids in due course.