DO NOT complain about balance, we've got bigger issues to fry
Yes, don't complain about certain issues AT ALL until ALL OTHER issues are taken care of. That is some twisted logic right there. The devs appreciate all feedback they get and they will sort through it to fix what they think needs fixing in an order they think is best. There is no reason to limit any discussion on problems (perceived or real) on our part.
On topic, using the medium as a baseline, I think lights get more disadvantages then advantages compared to a heavy. Heavy loses the ability to mantle and has a slower run speed, and in return they get access to weapons no other body type can get and can equip medium weapons in their secondary slot meaning you get a boatload of weapon loadout choices and choices are good. And they also get a lot of health, which is nice. Meanwhile what the light gains in speed and the ability to wall hop, they lose out in terms of weapon selection, meaning you have to go SMG/Pistol or Light Rifle/Pistol (or Pistol/Pistol if you really feel like it). And they tend to drop like flies due to low health. A Medium can equip Assault Rifle/SMG, Grenade Launcher/SMG. Shotgun/SMG, Assault Rifle/Light Rifle, Grenade Launcher/Light Rifle, Shotgun/Light Rifle, Assault Rifle/Pistol, Grenade Launcher/Pistol, Shotgun/Pistol and any of 3 combinations a light can take. That's 12 different combinations compared to just the 3 a light gets.
In the 25 or so hours I've played Brink, I really haven't found a whole lot of places a light can get to that other body types can't, at least not enough to worth losing out on the extra weapon selection. And even when you do find that one path you can go, it generally means you are taking that path alone. And chances are you'll run into an enemy or 3 and get murdered because you didn't have backup due to the fact that the game was designed to to discourage "lone wolf" behavior.
I've been forcing myself to go light for the past 10 hours of play just to see if I can make it work. And while I can, I wish I had more of a weapon selection as it is the only thing I really don't like about lights. I think making the shotgun a light weapon or making a light variant shotgun would go a long way about making light loadouts not seem so stale.
Part of the point with the light body type is that they have severely restricted weapon selection but tremendous options with movement. If they had more weapons it would reduce the benefits of the medium and heavy body types (unless everyone was given more weapons and more weapon types).
Any more light weapons that are added can immediately be equipped by any body type which would also increase their number of unique loadouts.