Sram (x and esp series) have 1:1 actuation ratio, where Shimano has a 2:1 ratio, therefore you cannot mix Shimano with Sram
The pro's riding Sram are probably getting paid to use the product, just as Shimano pays their athletes to do the same.
I think Sram is starting to take back some of the market share which is why you are starting to see more Sram products on more bikes.
Is the Sram better than Shimano? Both are putting out really good products (I do love Shimano's cassettes, chains, and rings better) and I have to admit, Srams X shifter/deraileurs do seem a lot more positive and less prone to mis-shifts when conditions are less than ideal
My $0.02