Whether it's a /24 for f-root or a /20 doesn't really make a difference- it's a routing table entry either way- and why waste addresses.vixie had a fun discussion about anycast and dns... something about him being sad/sorry about making everyone have to carry a /24 for f-root everywhere.
I think there are a few services where these sorts of exceptions make sense and f-root is certainly one of them.
-Don
