The thing is, if we believe we are on the side of reason, the nature of such an argument should only be considered a nuisance to us rather than catastrophic. If I want to prove that Israel is fascist and someone hits me with the "not everyone you disagree with is a fascist" line, then I can simply ask them to give me a set of criteria and either argue the criteria being incorrect or argue that Israel meets it (depending on the criteria, context, etc.) What our comrade is proposing is a way of opening discussion so that preconceived notions can be challenged more thoroughly.

Socialism is the ruthless criticism of all that exists; if a socialist can't produce a justification for their ideology, this isn't an argument they should be getting into (they should be studying, whether through reading or investigating the world).