Night sights are not absolutely necessary, but are useful for personal defense under certain low-light conditions. Most law enforcement officers and procurement officials think they are important enough to spend the extra money to get them, whether it's personal money or the taxpayer's cash.
Fiber optic sights seem to be a little more prone to breakage if banged into a solid object. Sometimes the light-gathering tube breaks or falls off, requiring replacement. Night sights get dimmer over time, and will require replacement every 7-12 years, depending on model and color. They can also be broken, but it usually requires more of an impact to damage night sights.
Many gun stores offer basic cleaning kits, sold by caliber, for around $20.