I often think that "Jaati-Gotra" might be having used by the Pandits for keeping easily Records of their clients.
Here by I wanna tell that in Haridwar (may be at some other places in India also),Pandas have kept record of births and deaths of many many many ancestors of you and me.They have recorded number of their family members along with their names etc.
They have distributed this Record keeping work amongst themselves according to certain Gotras of their Clients (called Yajmaan).
Many times ,I think that this labeling of "Jaat-Gotra" would have been supported because of this purpose also.This might not be the only reason but One might be one strong reason out of so many other reasons of this distribution.
Just like Cows/pigs are stamped by their keepers to avoid mixing with others.
