Rick ... I think you raise a very valid question. I considered this before ever buying one. After getting a ticket at night though, my whole perspective was changed. I went from not having one for a year -> having a cheap one and enjoying it -> getting one that will help me out just that extra bit more.
Reasons to have one:
- Ability to "see" cops at night.
- More awareness of hidden cops ... I don't know about you but here in NY, they looove to hide. Often I won't know until the detector goes off ... I start looking ... locate them and go "damn", I would have never seen him there.
- A small amount of reassurance on days with an open back road and no signal anywhere.
Reasons NOT to have one:
- If you are a person who constantly drives way above the limit because with a detector, you will start to think of yourself as invincible. People such as these will inevitably get nailed at one point or another and taught a very expensive lesson.
- Added reason to have car broken into (besides stereo).
- Good luck getting a warning: I heard somewhere that getting a ticket from a cop while having a radar detector visible is like giving them the middle finger and announcing I LOVE TO SPEED!
Keep in mind that I am not a person who speeds excessively, but I do enjoy my car.

... I have listed a few pros and cons but in the end, this is my own reasoning and I just enjoy having the detector. That's it, that's all.