Grooves are added to help release the gases produced by the pads vapourising.
Dents or holes also help break up the vapours - but as mentioned above sometimes this can cause the discs to be more susceptible to cracking. Pesumably, this is a design / manufacturing problem..... if the holes are radiused sufficiently they shouldn't cause the discs to crack.
These gases are the main part of what causes 'brake fade', the pads end up running on the vapours rather than being in contact with the disc itself - a bit like tyres aquaplaning on a wet road - the pads effectively start "vapourplaning". This is obviously a direct result of how hot ur brakes get.
The less organic material in ur pads the less vapour they will produce.
In theory, bigger discs shouldn't run as hot (as there is a bigger mass of metal to heat up and a larger surface area to dissipate the heat) so you shouldn't get as much vapourisation.
Disc grooves also help in preventing glazing of ur pads.
Grooves are a good thing for helping shift these gases, but do reduce the surface area of the disc for braking and will also increase pad wear and brake noise.
Horses for courses I guess.
Cheers
GeeTee 8)