To the arguers, sometimes simplicity is key, other times it is not. For example, nearly every professional company in the world has a very simplistic logo that everyone would recognize. In other cases, such as making a scene in a movie that involves destroying an entire city, I would hope for some **** good effects.

Look at the SF weapon bombs:
Gold M4 - Changed the color of part of the M4
Dragon M4 - Added a simplistic dragon design
Alcad M4 - Again, changed the color
Camo M4 - Added a simplistic camo pattern

You can make the most complex design ever and put it on a gun, yet some people might hate it. It might override the natural color of the gun and hurt people's eyes. Or you can change the color of parts and call it a brand new skin. Again, some people might hate it because it wasn't modified enough.

Personally, I think Top did a decent job on the M4 considering it has more design than most weapon bombs currently on SF while not covering every square inch of the gun. The key in this case is balance, it isn't a company logo or movie effects, just a gun on a very low-graphic game. Having amazing graphics on a gun then looking at a pixelated wall of grey doesn't seem right, so in this case gun graphics should lean slightly more to the simplistic side.