Of course I understand what a portrait grip is for - what I don't understand is their popularity amongst amateurs.
I guess my hypothesis is that big camera + big grip is too often a status symbol type of a thing, and a smaller and lighter setup will be perfectly effective and produce equivalent results. I'd like to see people overcome the status anxiety they seem to have when choosing a camera.
Hmm. I usually agree with most everything you say, firestarter, but this time I'm on a different page than you are. Seems to me that most 'amateurs' are all about portability. They're trying to get something as close to their pocket PnS as possible. The decision to append an expensive and bulky grip onto a camera is usually borne out of a need for better ergonomics (or better battery life, though that's less of an issue these days).
For example, I have one handholding camera and one for tripod use. The former is outfitted with a battery grip and the latter with an L-bracket. Occasionally, though, I will use the 'tripod' camera in my hands, and then I simply loathe taking photos in portrait orientation. Jacking one elbow up in the air is unstable, uncomfortable, and, well...unbecoming of a lady.

I may not be the best example of an 'amateur', since photography generates at least a third of my income, but nonetheless, I have far better reasons than "status" to opt for a grip. If looking 'big' were that important, I'd have grips on both cameras, but I have not done that--and I won't unless I start handholding the 5D2 more often.
BTW, to the OP: DPreview.com has just published their review of the D7000 and includes some comparisons with the D300s. Perhaps worth considering.