They don't evoke their rights as much as they should. It's much harder for police if they remain silent because the police has nothing to work with and/or use against them.
personally I do not think in most cases it matters if you have the evidence against them the courts will convict if not they will walk before they get to court
They evoke the right whenever they need to. it is harder to convict them because you don't have any information from them.