An old adage in the computer industry holds that dust bunnies inside of a computer are lucky. Let's consider some of the realities of dust bunnies:
Significant buildup of dust can restrict airflow inside the system, leading to increased levels of heat.
Dust can contain metal particles, which, in sufficient quantities, can contribute to signal degradation or shorts on circuit boards.
Put critical servers in a filtered environment, and clean out dust bunnies on a regular schedule.