[CharlesColeman]

> The phrase always originates with an accusation by party A that party B calls party C useful idiots, but there’s never any evidence backing up the accusation against party B.

I think the criteria "that party B calls party C" is actually pretty irrelevant to the concept's usefulness.

> essentially an argument meant to invalidate popular support for party B

In most of the cases where I'd use the term "useful idiot" the "party B" doesn't have much popular support, but is rather engaging in manipulation, disinformation, dishonest propaganda, etc. The realization isn't useful as a dismissal, but rather as a reminder that you need to confront both the idiot's sincere belief along with their ignorance of it's goals, implications, and beneficiaries.