XT level thumb shifters are not readily available and they go for ridiculously high prices on eBay. I looked for some at a price I was willing to pay for weeks before deciding to buy the used bar end shifters for $20. I already had the Suntour shifters. They came off of my MB-2.
I was also afraid that if I used the thumbshifters, I won't be able to get parts for them or replace them five years from now. It's at least a possibility with current production bar end shifters.
Besides, anybody can bolt on thumb shifters. I have something unique.