In my mind, and I may be missing something, if a resolver does not treat
the zone as unsigned, it will be making validation decisions based on
unverified data. Which, I think, is a bad idea. My memory is a bit
hazy on the subject, but wasn't it that sort of thing that caused us to
do the typecode rollover in the first place?
The security policy may say otherwise.
Say you have a policy that says "Every zone beneath example.net will
be signed with algorithm A" and example.net is only signed with
algorithm B.
Treating the zone as unsigned would be a violation of that
policy.