I have not seen a lot of RDAs with the extended function (EF) license... probably because it costs too much money (a couple of thousands for the preparation and the examination) to obtain the EF license and the salary is not that much better. One of my ortho RDAs has the EF license. I pay her the same as I pay my other RDAs.
If you do a lot of crown preps in a day (ie working for Western Dental), having a RDAEF is a huge plus because he/she can pack cord, take impression, make and cement temporary crown for you. The function of the RDAEF is still very limited. I think it is more cost effective for the owner dentist to hire an associate dentist (or a hygienist) who can do a lot more.