I buy auto versions for my builds for three reasons:
1) As a rule, with rifles based on military rifles, I try not to diverge from the military design unless its required. There is no particular reason to have a semiauto-only carrier, so I don't get one. It was the original, "real" design; why deviate if you don't have to?
2) Some say that the slightly heavier weight of the auto carrier over a semi with more metal cut off of it aids slightly with reliable chambering. I've never seen any documentation that proves this, and doubt it is ever a significant factor, but, who knows...
3) If I ever hit the lottery and buy a full auto lower, my uppers will be compatible from the get-go.
If, however, I bought an upper that happened to come with a semi-auto carrier, I don't think I'd consider it worth the money to replace it.