- Honesty is the best policy.
- The bubbles in bread are yeast burps.
- Maintaining your stuff will make it last longer.
- Bike stuff is getting more expensiver all the time. Cyclocross tires (!!!) are more than ATB tires, for heaven's sake.
- Maintaining your stuff will make it last longer.
- Motorex suspension fluids are very nice.
- The length of time for which the stuff of you lasts is in direct proportion to the amount of maintenance you throw at it.
Wait, no! The point is that your fork is an expensive, complicated thing. Retail on a middle-of-the-road fork (as in, 4 pounds, 26" wheel) is still going to be $400 to $500. For a nice fork, think $800-$1000. Inside, there are springs, dampers, liquids, protons, valves, nitrogen, mouse droppings, radio waves, shims, gases, and (if it's a Magura) the faintest hint of schnitzel.
Just like anything else, there are parts in this system that break down over time. For example, the top seals usually last about a season before they start to either weep oil or allow dirt inside the fork. The foam ring that keeps oil on your upper tubes gets corrupted faster than that. Damper oil has a shelf life, and can migrate past o-rings to places it doesn't belong. Speaking of o-rings, they are wear parts as well, and can begin to leak air and fluid.
What will you notice while these things are happening? Probably not much. Remember, you're the one who has been wearing the parts out, so it's likely you won't notice that much has changed because most of these parts don't puke in the middle of a ride. They wear out slowly, and eventually your fork stops doing anything productive besides weighing the front of your bike down. At least you won't notice until you get on a bike with a new or recently-serviced fork. There you will notice that the compression stroke is smoother, you get full travel, the rebound feels better, and all the adjustments turn easily.
Forks are sensitive to neglect, and performance can deteriorate without you even knowing it's happening. If you're racing, you need your fork to be working properly, or else you'd be riding rigid and saving the weight. If you're just riding, you want your stuff to work right. Don't wait for black goop to come out of your top seals. Don't wait for all of the oil to leak out on your garage floor. Don't wait until your compression stroke feels like your unshaved legs in late September and you can't hold a good line in corners anymore.
Changing the oil in your car is always cheaper than changing the engine in your car. Changing your the filter in your furnace is always cheaper than changing your furnace. Changing the diapers on your children is always cheaper than... uh... something.
Preventative fork overhauls are always cheaper than "my fork is leaking and clunking" overhauls.