RandomArray got a "special" position as part of Numeric simply by historical accident in being there first. I think in the conversion to Numarray we will be able to remove such things from the "core" and make more of a marketplace of equals for the "addons". As it is now there is some implication that somehow one is "better" than the other, which is unjustified either mathematically or in the sense of design.
RNG's design is based on my experience with large codes needing many independent streams. The mathematics is from a well-tested Cray algorithm. I'm sure it could use fluffing up but a good case can be made for it.