[gnicholas]

> As long as defense attorneys are involved, what is the harm in this?

Good point, suspects have a right to an attorney and the right to remain silent. But much of the successful lying probably happens when an attorney isn't present. For example "look, we can wait for your lawyer to get here, but this is looking really bad for you. We just arrested your buddy and once he sings we'll just throw the book at you. If you give your side of the story first, you get the better deal."

A suspect without a lawyer might waive the right to counsel/silence and incriminate himself. Of course, he would be guilty, so it's not entirely clear this is a bad thing (unless he is falsely admitting guilt, which does occasionally happen).