Themes
The concept spreads in variants of both directions. A pre-processor host would do the filtering of the warez/spammer/script kiddie signups. Say in three months out of 1000 total signups, only 150 are any good. It's not clear who does the migration - my instinct says it's the new Gold Host, who I expect to have the stake in the long term hosting. The new host then is spared the grunt work that has threatened to siphon off working hours.
Client side, if they know that certain hosts use pre-processor filters, it might deter abuse all by itself, in that there is less incentive for the "one week spam and run" types. I expect very few "sleepers" who play nice until the switch - the low end bad signups don't have the patience or savvy for that. Sure, you might get a couple jerks, but that's a "higher level problem" than what we have seen typically causing the frustration here.
Even giving the client the benefit of the doubt, they sign up legit, start a blog, post two entries and a picture of their dog, ... then wander away... those "dabble" clients are an important group to give them good service, but it's still a ghost account that a gold host doesn't really want. People just get busy, and wander away, but if they remember the decent service at the time, the second time they comne back might be the good client for an upsell account.
The last angle for now, is it gives an outlet for those people who want to dabble on the hosting side for the learning experience. We have disparaged them *because the distinction between pre-processor and gold host* is muddied. A pre-processor host who only operates for some five months can still produce five sweeps of filtered clients for a gold host. I'm pretty sure a client would have some fun signing up with someone trying out their very first host experiment. The client knows to expect a little bumpier experience. Meanwhile let's say bad stuff does happen, it's the only way the new learner host can get the experience that will later let them become a gold host. It's a measured "apprenticeship" path so that one bad unlucky mistake doesn't sink their reputation.