I'm guilty of it, too, but like +Anthony Russo
said, I try to keep it under control and to a minimum. As +Robert Scoble
has said, G+ needs better noise control so you can simply click a "block similar posts" or something like that.
That said, I really don't care that much for being required to share the contest post in order to enter. I'd much rather a simple "comment on the post in the required format and you're entered" method. But ... I can see why the websites in question would want to do that, as well; they (in theory) get their site publicized much faster and pick up more readers, etc.
And I see a couple people above posted that it's against G+ TOS to do contests ... I'm not aware of that, and would be interested in a link to where it says that. I haven't read all of the TOS, so I could quite easily just not know about that section; I'm not saying I disbelieve them. ;)