ground fighting is definitely something any martial artist should learn but having been doing it for a little bit. i dont think it is wise to take it to the ground in a real life situation and should only be used if it has to be
i think it is agood to know both grappling arts and striking arts. in self defense you never if your opponent will tackle you or just kick and punch you.
i know more of striking arts though. so technically, no it is not a seccondary martial art.
Brazilian Jiu Jitsu is not a secondary martial art, but it's not an end in itself, either. If you're going to learn how to fight, it is a great form to take, as it can end a fight rather quickly and decisively.
You're assuming that people study grappling for the purposes of using it offensively, in other words pursuing a takedown and turning the fight into a grappling match. However, it has just as much use, perhaps moreso, as a defensive tool. BJJ has taught me how to escape from underneath an opponent and how to fight off simple chokes, both by using solid mechanical principles. I would certainly agree that on the whole, grappling as an offensive strategy is best suited as a back-up, but I would consider bottom escapes, guard sweeps, and choke counters (as well as basic counters to throws and takedowns) to be as important as offensive striking and punch defense is.