there was a problem initially where any body could use any gun. this makes lights too deadly. (i found out by mistake and had a light with a minigun!). the different guns for different sizes helps balancing.
if you like lights you can use knife and pistol as secondary - thats lots of fun.
btw - most of the AR guns (rhett, gerund etc) arent much better than the SMGs. in many circumstances the SMGs are better (i.e. close range)
by the way - welcome to brink

hope you like the game. it must be a bit intimidating for newbies when a lot of us know the game inside out now. best play on the new maps at first - we dont, know them very well yet. i have only played 1 of them online and i still run around getting lost!