It's not a frequent occurrence. Once someone is senior enough to have earned a sabbatical, they've also reached a point where there are a lot of people champing at the bit for their job. It ties into the fact this is a young person's business.
Besides that youthfulness, why else don't you see them?
Client satisfaction can hinge on the continuity of people working on the business, and if a senior person is away for periods, agencies may think that's bad for the account. Of course, that may be perception more than reality.
For those that do offer them, do these leaves offer much of a point of differentiation?
Anything like that, whether or not it's something people can actually take advantage of, helps with recruitment and retention. ... It's kind of a love-it-or-leave-it business at the end of the day.