[3] Economist Murray Rothbard thought that by emphasizing marginal scenarios, Defending the Undefendable "does far more to demonstrate the workability and morality of the free market than a dozen sober tomes on more respectable industries and activities.

"[4] Libertarian activist Sharon Presley had a more critical take, agreeing with large parts of it but calling it "a bizarre combination of both excellent and horrible elements".

She offered some praise for its "valid economic and political analysis" but also criticized its "sensationalistic style", several instances of "faulty logic", and its overly "mechanistic and insensitive view of human behavior".

She feared it might ultimately act to discredit the ideas it was meant to promote, concluding that it is "a positive menace to the libertarian movement and dramatically demonstrates Rand's statement that the worst enemies of capitalism are its defenders.

[7] The philosopher Tibor Machan, who generally shared Block's libertarian leanings, wrote that the book "defends some of the silliest ideas in support of an essentially good cause...