Why on earth would anyone use a product that not only can't be painted over but also has no real stiffening ability. Silicone adhesive sealants are made by design to remain flexable.
Fin Fillets are intended to strengthen and streamline the joint. I wouldn't use spackle either but at least that product hardens to a someway rigid mass, that could be painted over.
Personally I prefer 5 or 30minute epoxy fillets, as they add strength to the joint, remain slightly flexable to absorb a little flex on hard landings and can be sanded, polished and/or painted over.