(defmethod a-method ((x t))
complains about x being unused, but it's used for specializing. All other implementations don't signal warnings in such case, and some signal only when the lambda expression is just (x).
So, I'd argue that ECL should not signal a warning in ((x specializer)) case, because it's a common practice and many programs assume it, which results in a lot of irrelevant warnings when compiling with ECL.
Another issue with this is that when that warning is signaled, it's printed as
;;; in file ...
But it's not a full warning, just a style-warning, so printing "Style-warning:" instead would be less confusing.