The harmonic balancer is there to 'absorb' some of the shock placed on the crankshaft by the combustion forcing the connecting rods down and ultimately around. Someone posted a study on this a month or so ago, stating that the crankshaft (obviously) flexes a bit every time this combustion event occurs and of course flexes back in the opposite way. As it gets further from something to absorb that momentum/flex deal, it's obviously going to be flexing around a lot more, that is why there is a somewhat heavy flywheel at one end and a heavy rubber enforced crank pulley (harmonic balancer).
