The difference between your Light and your enemy's affects how much damage you deal to each other. If you are lower than your enemy, you deal less damage and take more, and vice versa. The attack value of your weapon also increases damage dealt, so a light 300 character with a Primary attack value of 290 does less damage than a character of the same light with a Primary attack value of 300. Why they think Light correlates to actual skill is beyond me, since Light on equipment drops is randomized, but there you have it. A lot of dumb people doesn't make them right, it just means the status quo is idiocy.