The purpose of using gasket... within the aftermarket world... is simply to accomodate slight warpage, pittin or non-perfect surface conditions that become existant after years of usage. In a "new" environment (ne castings, new surface conditions, etc, the need for gaskets becomes less, especialy while theis no liquid fluid to be sealing. The OEs typically didn't use exhaust gaskets becaue the surfacs were pretty much perfect, thu, not requiring exhaust manifold gaskets.
Tubular pipes (such as headers) tend to flex and vibrate and can benefit from using gasket for long-term sealing.
All RTV -- even the so-called "hi-temps" -- eventually reverts back to a liquid when subjected to long-term high-heat... it doesn't last forever!