Cross drilling was originally done as an attempt to release a gas that was generated by a very specific pad material under racing conditions.
I personally believe that it caught on strictly because of its cosmetic effects.
To help understand whether it is useful or not, stop and think about the physics involved with brakes. Brakes simply convert the kinetic energy of motion and mass into HEAT. The brake components recieve this energy in a short amount of time, then dissipate the energy by transferring that heat into the air as they cool. This means that the brake components, mostly the rotors or drums are a CONTAINER of that energy.
As the MASS of that container increases, there is more volume for containing that energy. Drilling holes in the rotor DECREASES the volume of mass that can be used to deal with the energy (heat.) This means that it is not as effective as a brake component. I'm quite sure that the engineers TOTALLY understood this when they cross drilled those first rotors, but the advantage of dissipating the gas emitted from the pad material outweighed the disadvantage of the loss of mass due to the drilling.
As with ANY engineering problem, there are multiple factors that must be considered and often compromised.
If you like the looks of these rotors and it is that important to you, I say go for it. If you are simply replacing rotors for functional purposes, you are wasting your money. I don't doubt that they work okay, but you are paying more money for a smaller HEAT CONTAINER.