DVD9 is not always necessary, and is simply too expensive for general use.
Maybe for general use. But the burners are very cheap now and the discs will come down in price, just as DVD5 discs, and CDRs before them. have.
All the time there are people here who will spend a few hundred dollars on a new LD player for any improvement in quality, no matter how slight, I'm sure there will be people who will pay the extra couple of dollars for a blank DL disc.
Personally I'm happy with DVD5, but if you're striving for the best possible it seems a bit silly to stop just short of what's possible.