Spark plugs are one of the most neglected components on any motorcycle - until they go wrong, which happens eventually. Like many parts, they have a finite life, which is usually much longer than the recommended service interval.  However, regular replacement will pay dividends in the long run, so it is worth taking a peep at them every now and again, changing them for a reputable replacement before they fail.  It is much easier to change them in your nice warm, dry garage than it is by the side of a road in the dark and rain with trucks hurtling past...