According to a Westlaw search I conducted this morning, in the last three years there were 506 cases that cited to the Restatement (Second) of Agency. In that same period, there were 630 cases that cited to the Restatement (Third). Ignoring the fact that some cases cited both, that means over 44.5% of cases in the last three years cited a 63 year old Restatement instead of citing a 15 year old Restatement. I assume that a large part of the persistence of the Restatement (Second) is driven by lower courts following older higher court precedents that embraced the Restatement (Second). Eyeballing the results seems to confirm that. Yet, there were a fair number of state supreme court cases citing the Restatement (Second).
It might be interesting for someone with a lot of time on their hands to drill down into these results to see if there are doctrinal reasons courts are sticking with the Restatement (Second).
It also suggests that my fellow teachers of agency law cannot yet throw the Restatement (Second) into the dustbin of history.