Often seemingly self-evident primary sources are not reliable. In fact very often the converse is the case. They are explicitly unreliable, erroneous, biased, slanted, POV or downright lies and propaganda. Even basic level history students are taught that in evaluating evidence one has to be critical of its provenance, its intention and the careful handling of all such testimony as corroboration. The current obsession with citing sources has led to rafts and reams of citations which fulfill policy criteria at the marked expense of quality.
That was exactly the point I tried to make in the other thread at this talk page. The use of primary sources should be discouraged when the contributor doesn't have extensive knowledge about a subject. In practice, that means only specialist users should be using primary sources. Other users should be actively discouraged to do so.