How sad are reunion tours? I suppose the spectrum is broad — between the goulish spectacle of another Rolling Stones rollout and the mostly respectable showing by folks like the Pixies — but still it's just an unnecessary exercise. Unless you need the money, in which case, put that disclaimer on the name of the tour, so everyone knows it's about real need and not vanity. Which is why Os Mutantes's wanna-be arena rock showing at Prospect Park this last Friday was such a bummer. It's not like they were ever in it for the money. And really, the only thing you can do to legendary status is to tarnish it by putting your good name on uninspired mediocrity in your plump years. At least it was another beautiful park night, and happily, some kids in the opening band saved the day with their giddily backward-looking sugar pop. Nothing for the ages, but great for welcoming a summer Saturday night.