if you rotate tyres, you may get more even tyre wear, but you don't have the benefit of the best traction. for instance, if you wear a slight slope across a tyre, it will give you a good footprint on the road. if you then put it on another position, yes it might even out the wear, but it will initially wear away the highest part of the tread. it won't have the best traction until it wears the high part down to the level of the rest of the tyre. (thus it not only has less than 100% traction but it has also worn more rubber than necessary off the 'high' part of the tyre.)
years ago, car service schedules used to have a diagram showing the correct order of 'rotation' so that tyre wear was evened out, including the spare, and you had to buy 5 tyres at a time. nowadays, most manufacturers suggest leaving a tyre in one position until it wears down to replacement level, then buy two new tyres and put them on the BACK (regardless of front or rear wheel drive). with directional and 'handed' tyres it's only possible to move front to back, not side to side.
clear as mud, eh? :)