As more people have pointed out, the advantage of the LT is in its speed and maneuverability. This means 2 things IMO:
1. You can dodge shots more easily should the terrain allow a 'dodge shots advantage'. If there's enough cover to maneuver in the map, a LT might defeat a HT. If not, offcourse, the HT wins and a MT might be the better choice.
2. You're faster: Meaning that in a completely open scenario, LT's might get to their target faster than MT's. Suppose they get there 25% faster (I have no idea on the actual stat), but have the same DPS -- This would mean a 25% higher damage output! (P.S. then again, if you get there around the same time, and you receive return fire, the MT would hold our, say, 25% longer, making its damage output 25% higher!)
Same goes when you're on the attack, but notice that your base defense is going dramatically -- the LT would get back much faster.
So in other words, the LT and MT just have different uses in different circumstances. To each its niche. But that's just my view on things!