hii've got a slightly different opinion. Consider an airconditioner, when the compresor is on if you switch it off probably you could see a sprak build up in between the switch, well its because the airconditioner was workin in full load and thus drawing high current and that when it stops suddenly sparks are produced. Same is the case with pcs, when you turn off the pc using the power button the pc actually might've been workin in full power and a sudden stop could potentially damage smps/hdd/cd-dvd etc....on the other hand when you turn off your pc from the start menu each and every devices are switched off one by one and finally the main supply is cut off, now that does'nt cause any damage.