The issues is not one of dependencies, it is one of necessity. We would not enable proprietary drivers just for fun (they are already *there* just not used by default). We will only enable them if there is some free software functionality that is important and in turn depends on them. The closest we have to that is Compiz or Beryl (TuxRacer is fun but doesn't count as essential) and neither of these is yet ready to be included in Ubuntu by default. Therefor, there is no free software component of Ubuntu that depends on the drivers, therefor we don't enable them by default.