There are many reasons why you should get annual bike maintenance. A professional inspection of your bike can reveal safety and performance issues. This list reveals the most important reasons why annual bicycle maintenance is one expense you shouldn't skip.
Enjoyment - Riding a bike is fun. Riding a poorly maintained bike is less fun. Unsatisfactory (or unsafe) braking and shifting performance, untrue wheels, and other issues can really kill the fun.
Safety - Fact: bikes regularly inspected and tuned have a lower likelihood of chronic failure which could cause serious injury or death. Worn or corroded cables can break, and worn brake pads won't stop in an emergency. A quality bicycle repair shop will make sure all nuts and bolts are tightened to the proper spec and everything performs as it should.
Longevity - I can't find the original source, but there's a statistic that states something along the lines of bikes available from mass-merchant retailers last an average of 2 years while bikes from specialty retailers last an average of 15 years. The point here: bikes aren't disposable toys. Quality bikes will last longer. Even if you plan to do nothing more than ride around your neighborhood, you should steer clear of bikes from big-box stores. Most of these bikes are lower quality brands or models, and many aren't assembled properly to begin with. Yes, a $300 bike from a bike shop is (at least) twice as good as a $150 bike from Target. While quality is a factor, I think maintenance plays into the longevity statistic. People who pay more for a quality bike are more likely to invest in keeping it running properly.
Save money - Good bike mechanics can find and fix little problems before they become expensive to repair. For example, bearings in the headset (steering), wheels (hubs), or crank (bottom bracket) could damage a bike beyond repair or will need expensive replacement if ridden extensively when loose or contaminated.
Reliability - For those of you who ride your bikes longer distances, reliability is important. This is especially true if you rely on your bike for transportation. Bikes regularly inspected and tuned are less likely to leave the rider stranded on the side of the road.
Resale value - Resale value is often overlooked. Bikes that work well, especially brand-name bikes, have a very high resale value compared to bikes that are inoperable. Inspect your local classified ads or Craigslist and you'll immediately see that used bikes in great shape capture a premium.