The R5 is used because of the role an assault rifle plays in the team strategy. If there were other ARs that could perform at range with such accuracy as the R5 they would use it. There have been a few AKs and even SC2010s used but R5 is king.
You should try going up against these guys with any other assault rifle ! You'll quickly understand why you have to use the best available.
The MSBS was op, im sorry those who disagree but the stats are there.
I agree the MTAR is slightly op, but only because of its range.