I have a roll cage on my emaxx, and for that particular truck, the roll cage I believe made a big difference in the reliability of the truck in the fact that it tied the front and rear of the truck together to help against flex between the bulkheads and the main chassis. I didn't notice any difference in the performance of the truck, it's a monster truck, not a precise racing machine. The cage I have is made of stainless steel bar, so it's not really easily bent, it'll bend by hand, but it takes a fair amount of persuasion.
My experience with the roll cage I had on my emaxx may be quite different than other's experience, which it sounds like, was quite positive, and I feel it saved the truck a couple times.