How valuable are the insights of former "leftist" radicals to conservatism?

Very. They have first hand knowledge of how those views and goals are failed ones. The problem is the mindset that causes one to be a Leftist radical in the first place near allows one to truly leave it in the dust and become a former one. They are forever enamored with the utopian views of it and are always ignoring the pesky pragmatism that keeps it from happening.