The difference is IW just streamlined it and turned it into another "perk", so you don't have to deal with a bunch of random percentages. I really don't like leveling weapons/roles for stat boosts, but if I had to choose, I think I would take the perk system over RO2's leveling. All of the number crunching is done and all of the benefits are clear-cut.
That being said, I would rather have a game where the stats are the same across the board, and the only differences between ranks are their loadout options...probably a combination of BF2 and BC2's rank systems.
A perfect system IMO
-no vehicle unlocks
-weapons unlocked through progression
-some attachments for weapons unlocked through progression (weapons can be customized!)
-NO proficiency modifiers