The main thing that kept the cops from continuing to interrogate after they realized a person was electing to use their right to remain silent is a rule of exclusion, which is called "the fruit of the poisonous tree." Basically, if cops violate your constitutional rights and get evidence based on that violation, the evidence is inadmissible in court. It is a big deterrent because cops do not want to violate your rights, have you tell them where the body is buried, and then have any evidence gathered from the body held to be inadmissible in court. The Supreme Court put out such a harsh rule because cops were violating peoples Constitutional rights like mad, and something had to be done about it. Since the advent of the poisonous tree doctrine, cops are far more careful about violating a suspects rights, which is something every American should be grateful for.

As far as "clarity required" the previous default option was you were NOT waiving your rights. If you were completely silent for a long period of time where a reasonable person could recognize you were exercising your right to remain silent, the police had to stop. Now, there is no such thing stopping them. I know the point seems like a minor theoretical difference from the original position, but it has huge practical effects for those persons in custody. I could now make statements all day about I want to be silent, but without the magic words I may not get that right. Some cops will be good and realize it is time to back off, but for those who many not be as good the Supreme Court just gave them a golden ticket.