Rear derailleur/Sram XO vs. XTR

I see alot of pros use a all shimano components except the rear derailleur, they use sram xo or x9. Why is this? Is there an advantage to using sram over shimano on the rear? If so what is the advantage? Thanks.

 

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

 

It could also be that the pros dont like the STI levers. I know alot of DH riders refuse to ride them yet endorse them. O well. 2007 Shimano will start to offer both STI and Rapidfire again.