The problem is this can be more of a hassle for you. I've been pulled over as a teenager and when I said no he decided to search my car anyway. Saying no doesn't always help, in fact sometimes it makes them give you more trouble and search harder.
No, the police would like for you to think that. But imagine this scenario. You just dropped off a few friends. One may have dropped his prescription pain killer in between the seat cushions. Now, you've consented to the search and they found that dropped pill. You have no recourse, no prescription for a controlled substance and all you can say is it's not mine and you don't know how it got there.