i'm not saying that writing to dvd is the best way to record. not at all, but i think writing to a hdd is.
I may be mistaken, but my mini-dv cam transfers in realtime over firewire and usb. maybe i'm doing something wrong, but I've yet to find instructions on transferring it faster than realtime. I've also had my mini dv cam for a little over a year now, and just from normal use, i've already had to clean the heads on it.
Also, technically you could already tape onto dv - you just need a camera with audio imputs. You'd have to go line-in cuz i'm not aware of any cams that have digital in, but you could do it. my camera will tape 16/44.1 - however, only for a little over an hour, it'll do 12bit audio for an 1.5 hours. I know you're saying that if you modify a dat you could get just audio on the tape and not video, allowing for a lot more space. however, to me i seriously think that technology is moving away from "tape" storage and onto other things.
Also, you have to figure in the price of buying new tapes for every set. mini-dv does degenerate over time, so if you want to get the highest quality, you should use a new tape every time. Same as DAT tape. I've used my njb3's hdd over and over and over and over again. Infact, if I would have been buying DATs the whole time I had my njb3, I'd probably have a few hundred dollars worth of dats. To me, I'd rather spend the money on other equipment.