One way to look at it - a bad hub reflects on all of us. If it's a stinker, I vote it down without feeling bad about it. I think that a hub that has been published without preview before publishing and that still needs a lot of editing is a stinker. I put a lot of time and effort into each hub I write. I expect that other hubbers will show the same care.
Sometimes people make errors that aren't due to sloppiness, but are due to simply not knowing how to write that well. In that case, I consider whether or not they have something to say. If they don't, I vote the hub down. If they do, I copy the hub, and correct the errors for them. Then I email the corrected version to them. Then I suggest that they have someone else review their hubs before they publish them. I've only done that a few times but feel that, for a writer that could become better with help and practice, it's better than voting them down.