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.
FYI, the full-circle FA carriers simply have the dimensions to trip the sear in an M16/M4.
There are alot of people who justify use of FA carriers in AR-type rifles by using the old "heavier is better" excuse. In all my research and testing, the small amount of added mass in a FA carrier does little to aid chambering. Obviously, it does not impede it or hurt the process. Alot of what you hear is internet justification for a FA carrier. People just seem to want more M16/M4 parts in their weapons. IMHO, this grew out of earlier BATF issues with FA parts in a semi gun. When the BATF/Colt letter hit the internet, alot of people jumped on the bandwagon.
That being said, I only run FA carriers. If I ever obtain a sear/RR, then I'll be set. Like ny32182, I don't think it is worth trying to sell and then replace a semi half-circle carrier for a FA one just for aesthetics. All my AR's are Colt guns bought after 2005, so they all contain FA carriers.