Consider these two couplets:

Couplet #1: "Socialism has failed." "No, real socialism has never existed."

Couplet #2: "Libertarianism has failed." "No, real libertarianism has never existed."

In both cases, the point of the first clause is to discredit an economic system.

In both cases, the point of the second clause is to shield an idea.

And in both cases, the shielding comes at a high intellectual cost: You escape blame for real-world failures, but also lose credit for real-world successes.

Y Denies that Real X Exists

Strategically, then, you'd expect advocates of views with few successes and many failures to adore the "real X" defense. Advocates of views with ample successes and few failures, in contrast, will use it more reluctantly. This expectation holds up: Though both groups have been known to invoke the "real X" defense, socialists are far more likely to deny the relevance of actually-existing socialism than libertarians are to deny the relevance of actually-existing capitalism.