Still less, though, is it evidence for the presence of X.

Agreed, the evidence for the presence of X is that humans have been talking about it for a long time and seem to mean something.

Especially if the work a theory of X is supposed to do can be done without a theory of X, or turn out not to be necessary in the first place.

Careful, it's very easy to convince oneself that one doesn't need a theory of X when one is actually hiding X behind cached thoughts and sneaked in connotations. For example, Russell no doubt believed that he didn't need a theory of causality to do the work the theory of causality was supposed to do.