The hole is used to aline the pressure plate teeth with the gaps in the clutch centre grooves. If they are misalined, the teeth will partially rub against the clutch centre grooves, resulting in plates not pulling in fully and clutch slipping.
There is a wrong rotary position and there is a right rotary position, thats why the index hole is there.
Nothing to do with balancing.
If you count the teeth, there are 29 teeth. When you divide 29 into 360 degrees, you get 12.4 degrees between teeth. The spring bosses are 60 degrees, so you can rotate the pressure plate every 60 degrees. The problem is that 12.4 degrees does not divide evenly into 60 degrees. If yamaha had used 30 teeth, then the rotary position would have repeated every 60 degrees, and there would have been no need for the index hole. I think Yamaha used 29 teeth just to make it a little challenging for re-builds.