Better to say it MUST be valid at the outset. If they can't make
it work then
let the validators, marketplace and competing products shame them into
compliance.
+1.
As well as a validator, a series of test feeds should be created for
checking feed consumers, and everything set up to make it as easy as
possible to follow the spec. For a consumer to call themselves
spec-compliant, I suggest that they should at minimum display a warning when
invalid data is encountered. It would be too much to expect for all
consumers to behave like complete validators, but I think it reasonable to
demand basic checks such as XML well-formedness.