I think half of the reason for their great skill lies in the fact that majority if not all of the players play stock class paintball on a regular basis, against pumps as well as the fastest semis, and let me tell you that they can hold their own against the semi's. Stock play teaches the importance of movement into position since your rate of fire is very limited. You have to make your shots count. With this in mind, they become excellent movers and crawlers, so their movement skills are second to none, as well as their accuraccy. Now if you take those movement skills and you combine them with the rate of fire of a semi than you have one hell of a paintball player.
Stock guns are by no means a thing to laugh at at the field.
One of the guys that i heard about had moved from the islands,and went to play at a field somewhere on the mainland where even pump class was unheard of. He strolled out onto the field with his stock class paintgun, and most of the people on the field laughed at the fact that he had the guts to go out there with just a pump, much less a stock class pump. Well, much to their surprise, many of those people found themselves on the recieving end of that stock gun and soon found themselves walking off the field in bewilderment. The point i'm trying to make is that pumps, and stock class guns are no joke. They're a great addition to any players arsenal.