Isolating recursive from non-recursive servers has a ton of benefits:
1. measuring your external from internal queries becomes easier, hence
budgeting for the appropriate servers has a cost matching ability
2. to use distributed director from cisco, you need non-recursive
authoritative servers
3. your authoritative servers become less susceptible to corruption
from a looped delegation, hence isolating your DNS problems to
the recursive resolvers instead of taking down all your authoritative
abilities
etc. etc.
Rob