@RichieBamBam As far as I know, the only reason not to is really because you shouldn't have to. Having your opponent's arm crossed (unable to base) and the leg kicked out (unable to base), you should only need to simply roll them over. So trying to get your hook in might only give your opponent that much more time to figure out some way to escape. But if for some reason something changes and the sweep's not working, you can and should use your hook to finish the sweep.

Personally from what I've seen, the reason master Marcelo is able to sweep so effectively is because he doesn't give up on sweeps. If something doesn't work right away, he adjusts to something that will - even if that just means changing an angle or continuing to lift with a hook as high as he can.