Seriously, because if we don't have right to have a non-police witness of some kind present at our interrogation by the police, then in practice the cops pretty much have carte blanche to do whatever they want to people. Without a right to a lawyer during interrogation, who knows how many people will "fall down the stairs" while in custody.
I just moved back to Canada from Japan and while I love the Japanese culture and people and all my friends there, civil liberties in Japan are something of a bad joke. The court system has a conviction rate of nearly 100% for a couple of reasons. First, many criminals turn themselves in due to the shame-based nature of the social contract in Japanese culture. Second, there is no real presumption of innocence in the legal system. Third, the police can arrest and hold people without charge for days and the people being held have no right to a lawyer during interrogation.
In theory, this allows the police to avoid those cases where some crook gets off on a "technicality" because he wasn't read his rights or because he lawyered up and was told to act in his own best interest instead of the best interest of the state. In practice it means that when a crime is committed and the culprit isn't giftwrapped and waiting for them, the police in Japan can pick up anyone they consider undesirable, hold them incommunicado for a couple of days pending charges and give them the phone-book-and-rubber-hose treatment until they decide to confess. Usually, it doesn't even take any physical abuse. Three or four days without sleep, a complete ignorance of the rules the game is being played by and constant browbeating is usually enough to get the average homeless person or teen slacker to confess to whatever petty crime the police want them to in Japan. For more serious cases, the police can hold someone for a few weeks. After a couple of weeks, most people would be willing to confess to the McKinley assasination. And any accusations of mistreatment invariably boil down to "who are you going to believe - us, the sworn officers of the law who protect you while you sleep, or this whining criminal scumbag?"
The supreme court ruling sends us down the path to exactly that reality.