Your point is true - with most superzooms, you're lucky to start around F3.5, and that's at wide end...most fall into the F6 or smaller at full zoom...so it definitely won't compete with a nice F2.8 prime at the long end.
However, don't throw out the superzoom lens because they take on a whole new usefulness as you upgrade your camera body. On my last camera, my 18-250 was limited to daytime use, or the occasional long-exposure night shot...anything in low light was pointless, as I would be dealing with super-slow shutters and blur, or horrible noise from high ISO. But with my camera body now shooting clean ISO6400s, I can use the 18-250 even in low light situations, cloudy days, etc. Most of the new batches of cameras are very clean and detailed well into the high ISOs - 3200 at least, and often even 6400. So the overall versatility of the superzoom lenses depends on how well your camera can raise the ISO to get faster shutters to compensate for the lens' aperture limitations.