There are many more reasons than just these five, but they make a good start:
1. It's your constitutional right.
The 4th Amendment to the U.S. Constitution protects us against unreasonable searches and seizures. Unless police have strong evidence (probable cause) to believe you're involved in criminal activity, they need your permission to perform a search of you or your property.
You have the right to refuse random police searches anywhere and anytime, so long as you aren't crossing a border checkpoint or entering a secure facility like an airport. Don't be shy about standing up for your own privacy rights, especially when police are looking for evidence that could put you behind bars.
2. Refusing a search protects you if you end up in court.
It's always possible that police might search you anyway when you refuse to give consent, but that's no reason to say "yes" to the search. Basically, if there's any chance of evidence being found, agreeing to a search is like committing legal suicide, because it kills your case before you even get to court.
If you refuse a search, however, the officer will have to prove in court that there was probable cause to do a warrantless search. This will give your lawyer a good chance to win your case, but this only works if you said "no" to the search.
3. Saying "no" can prevent a search altogether.
Data on police searches are interesting, but they don't show how many searches didn't happen because a citizen said no. A non-search is a non-event that goes unrecorded, giving rise to a widespread misconception that police will always search with or without permission.
I know refusing searches works because I've been collecting stories from real police encounters. The reality is that police routinely ask for permission to search when they have absolutely no evidence of an actual crime. If you remain calm and say no, there's a good chance they'll back down, because it's a waste of time to do searches that won't hold up in court anyway.
4. Searches can waste your time and damage your property.
Do you have time to sit around while police rifle through your belongings? Police often spend 30 minutes or more on vehicle searches and even longer searching homes. You certainly can't count on officers to be careful with valuables or to put everything back where they found it. If you waive your 4th Amendment rights by agreeing to be searched, you will have few legal options if any property is damaged or missing after the search.
5. You never know what they'll find.
Are you 100 percent certain there's nothing illegal in your home or vehicle? You can never be too sure. A joint roach could stick to your shoe on the street and wind up on the floorboard. A careless acquaintance could have dropped a baggie behind the seat. Try telling a cop it isn't yours, and they'll just laugh and tell you to put your hands behind your back. If you agreed to the search, you can't challenge the evidence. But if you're innocent and you refused the search, your lawyer has a winnable case.