A reader who rides one of his bikes "probably less than 1000 miles a year" wanted to know if there was really "any compelling reason to change the oil every year? Why should I throw away 15 bucks' worth of oil that certainly isn't dirty or worn out?"
The answer is that you should change your oil at least once a year. When you ride the bike, small amounts of combustion by-products work past the rings introducing, among other contaminants, water into the crankcase. This mixes with the oil and gets distributed through the engine where it can begin to corrode unseen but expensive bits. If the bike is used just for short trips and rarely has the opportunity to get and stay hot for a half hour or more, the moisture will build and dilute the oil, diminishing its ability to lubricate, clean and cool. In fact, if your bike is used almost exclusively for short rides and rarely runs fully warm for extended periods, you should change the oil maybe three times a year. Ideally, you should get your engine up to operating temperature and keep it there for a half hour at least twice a month.
We recommend replacing the filter at every oil change (See How to Change Your Motorcycle's Oil and Filter). The stuff you don't want in your engine ends up in the filter. Why tempt fate? In fact, we suggest you cut open a used spin-on filter, spread out the pleated element, clean the debris with solvent (use a rag as a strainer), and check for signs (like metal filings) of problems.