One other benefit: a lot of people, myself included, would rather buy an app that is "officially approved" by Apple, even if it means paying a little bit more than I would getting it directly from a developer on a totally open platform.
Another benefit is the flipside of this; the quality of the large sea of
free software will put pressure on the non-free software. This'll be interesting because it's relatively rare that a new platform comes into existence, goes through a year of having only free (albeit black-market) software available, and suddenly non-free apps are competing with the pre-existing free apps. (In contrast, e.g., to the situation on desktops -- OpenOffice, AFAIK, has always been a free alternative to pre-existing commercial software; GAIM and Adium and the like primarily came into existence to provide alternatives to preexisting commercial products, etc).
I comply with the law, but I'm not interested in Apple's blessing until the point where I'm convinced that it will make my phone work better. Since I already make use of unlocking, I already have the infrastructure (Installer.app) to use contraband software.
So on the flipside, my expectations of SDK software are high. If it's mediocre, I won't even think about paying for it, much less paying a premium.
EDIT: BTW, are developers under complete NDA in terms of talking about the SDK software they're developing? How does that work? It seems to me I have seen little if anything about what to expect from SDK software, except the reviews of the development environment itself that basically pointed out all the things the software would
not be able to do.