I don't like spam hubs, but I think they should stay away from 'invite only' and focus on discouraging spam. I'm a hard worker, I write good hubs, and I bring in a lot of traffic, but I doubt anyone would have ever invited me.
I'd be more concerned about losing out on a good writer than carrying a bit of dead weight if I were HP. It's a little more work but it's also good for PR and friendly to new writers like myself. I don't think HP would ever go closed writing, but with the number of spammers around it's hard to know for sure.