The SMART system is what makes Brink, without it there would be a few advantages like vaulting, but the maps would become progressively smaller because you can’t mantle.
Also, the added walking time to reach the frontlines would cut down on experience points per hour in heavy-only gametypes. Medium gametypes would receive the same xp as the regular gametype on average, but similarly to elimination/search and destroy, you would actually have to give an xp multiplier just to be able to level at the same rate for heavy only.
Light only according to the poles would have approximately 4x more players than heavy only according to the poll (http://www.splashdamage.com/forums/showthread.php?t=18597), which I admit is a biased poll, but the Bethesda one (http://forums.bethsoft.com/index.php?/topic/1081398-character-which-size-will-you-pick/) shows nearly the same correlation, so light-only would cater to more players. Medium would cater to about the same amount of people, but at the same time, the difference between a medium and either light or heavy is not great enough to create a new gametype, because ~40% of the players are going to be medium anyway, thus medium fighters are often close to their comfort zone, even if there is a little variation than would be true of a medium only gametype.
Plus, if there is a light-only gametype, there might be a larger crowd of snipers who can get 1hit kills with an engie buff (assuming their enemy is not over-healed, which could rarely be achieved before without sight of a damaged enemy). Then there are knifers, which seem to be a growing presence in FPSs. They legitimately set up running classes just to knife people. They would favor this gametype as well.
Also, for the more hardcore SD fans, a light-only gametype in Brink would have a fast-paced gametype that is similar to ET, even if the HS kills are different than one of the two games (can’t remember which). Two of my friends had apparently played W:ET and loved the game (I found this out after mentioning Brink was made by SD), but they only play multiplayer on console now (one Xbox, the other PS3). Brink is different than both W:ET and ET:QW, so it might not be as much fun if the pace is slower. By adding a fast gametype, you attract more console players that liked playing W:ET in the past.
Overall, the reasons for light only as opposed to med/heavy are the SMART system limitations, less xp and a smaller crowd being catered to (whether snipers, knifers or ET fans that have moved on to console).


