Do I need a skimmer?

skimmerDo I need a skimmer for my basic mixed reef tank? For our KIS system, the answer is no. The negatives include initial expense, noise, electricity consumption, heat, and possible leaks.

For us, there is no substitute for water changes, so we removed the skimmer and have not looked back.

Exactly what does a skimmer remove?  The bad stuff?  Some good stuff?