The same amount of data is trivial to handle with rbldns. This isn't rocket science. Whether caching-only servers are more common or not is irrelevant. On the contrary. The installation, configuration, and maintenance advantages of djbdns are particularly obvious for pure caches.
Some sites run pure caches on practically every workstation.
This is a large part of the DNS market, and it's not an easy one to accurately survey. But my question was a different one. Anyway, as you already know, tinydns supports all record types. My top priority for djbdns is to implement an anti-forgery system that actually works. For replication, ssh and rsync do a much better job.