IMO it has to do with growth.
Google likes to see natural growth.
Adding 100 links in one day is hardly natural growth.
Also, (no I have no base for this other than years of personal observation) I think google has groupings of quality of sites.
New, barely indexed sites on the bottom, and so forth. I think that google lets you do grey things in degrees. Group one at the bottom gets to get away with a lot of stuff. When you move up a notch, it’s a bit stricter, and so on.
This would keep sites at certain levels depending on how grey or white they are, as far as bots determine that.
Before you reach the highest group (authority sites or whatever you want to call it) you get a real person looking you over. At that point you better be lily white and your history better be one of continued whitening.
I don’t think google outright penalizes most grey stuff, they just set a limit for how far up you can get with it. It makes for natural selection, the dark grey ones get frustrated and drop out of the game. Medium grey never passes a certain stage.
Page rank is not an indicator of this at all.
Thats just what I think, I don’t even read SEO forums. I occasionally (maybe once a month) read Matt’s blog. So go ahead and shoot it down, but it remains my own observation.
Of course, if you are all black, sites are disposable and none of this matters at all. You shoot up, and hope you stay long enough to make some money before you get removed.