The peculiar grammar of comparative clauses

THIS intriguing grammar question was raised in Jose Carillo’s Forum recently by a member in Russia who goes by the username Ivan Ivanov:

“I have a small question. Some grammarians write in their books that there are four types of dependent clauses: noun clauses, relative clauses, adverbial clauses, and comparative clauses. Is there any good reason indeed for classifying comparative clauses as a special type of clause?”