This happens in 6.8.0 as well, for some reason only the Fusion and Windows alternate styles are available, no gtk2/gtk3. I had no problems with ~5.8, but I USED TO have that problem with that version back in the day. It wasn't a happy experience let me tell ya :3
The problem is, as Qt is being developed, the closer you are to the 'bleeding edge' versions, the less items are compiled in or ported to that version so if you are wanting full environmental compatibility you might want to stay back a few versions to avoid this. The only reason you'd really need to hang out near the bleeding edge is if there are features you program "just has to have" in order to work.
You can turn these on and compile them yourself, yes, but don't be surprised when you run into errors (and I would expect there to be) that you will have to sort out yourself until they are officially done or possibly get someone else to do it (offer them food, that works sometimes :3)
Since the themes are really last-hat, they won't get to those until every other thing is working smoothly, and you may have to wait until other versions 'catch up'.
I remember waiting a considerable time for 5.8 and compiled it myself spending far too much time patching, and wondering if it was worth it--in the end other things forced me back to earlier versions anyway (specifically some older KDevelop controls and stuff wasn't integrating with the new Qt). So be sure to take the time to think about everything you would have to gain or lose before going ahead to the newest/latest versions and whether or not it would be worth it for you in the long run. :3
