Bribery laws also don't mean that every crooked official gets caught, but still they have some positive effect as a deterrent.
I'm fairly sure that if a cop had a 'less than 10 percent of the time' risk of permanently ending his career, then they would be much more careful than they are now. Of course, there could be an argument that we as a society don't want our cops to be too careful with rights of criminals, we also have an interest in them taking some risk and getting risky evidence; but that's a bit different direction of values/morals/rights.
I'm fairly sure that if a cop had a 'less than 10 percent of the time' risk of permanently ending his career, then they would be much more careful than they are now. Of course, there could be an argument that we as a society don't want our cops to be too careful with rights of criminals, we also have an interest in them taking some risk and getting risky evidence; but that's a bit different direction of values/morals/rights.