Just because OS X Mavericks had 8 DP's does not mean Yosemite has to stop at 8 as well. You have to take in mind that Mavericks had no UI overhaul at all, it was just feature focused. Mavericks needed less work than Yosemite.
There are rumours that there will be a DP 9 today or at 10th October but it's all speculation.
With each DP they have released, old bugs gets fixed and new ones arrive when they add in new stuff so if they made a GM for developers now and that had many bugs due to all the new stuff they add, it will just make things worse for them. When Yosemite is completely re-designed and everything looks great, then we can expect a GM. Probably late October.
Either way, Yosemite has still far to go to even enter GM state. Apple is not going to leave anything in the old design, they are re-designing everything, even the cursors like they added in DP8.
OS X Yosemite will not feel inconsistent when it releases, so if anything, we can expect more. Just like with iOS 7 and now that iOS 8 has released, Apple has taken engineers from the iOS team to help finish Yosemite in time for the Fall release.