A JPEG saved as a DNG isn't really a DNG file.
It is true that Aperture doesn't have "comprehensive" DNG support. It doesn't support linear DNGs, and it doesn't support some of the newest DNG extensions that Adobe has added (lossy compressed DNGs, etc.). I don't know the exact version of the spec that Aperture supports.
But do note that DNG is a lot of things. It's a fairly comprehensive spec that has many versions, with lots of different things. Aperture does support many DNG properties, but not all of them. If you want the latest, greatest, most comprehensive DNG support, then Adobe will have it, as it's their specification.
I personally just use the RAW files that come out of camera - whether they be NEF, CRW, CR2, or DNG. I _never_ convert my camera RAW files into DNG, because that actually strips some data (in my case, focus point informatin from CR2 files that Aperture can display, is removed from the converstion to DNG). There are many long-winded discussions on the "value" that DNG provides, and I don't subscribe to them. It provides no legay-proofness today that's unavailable other places.