I think generalizations, history etc. are quite valuable. Although it doesn't get mentioned much, MO answers serve a dual purpose: first of all, the answer should answer the OP's question. But after the question has been answered, it remains on the website, where the software conspires to make it turn up easily on Google. That means that answers (and questions) should also be thought of as archives for future questioners (and answerers). From that perspective I think the value of interesting side comments is huge.
(What is discouraged, I think, is soapboxing.)
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