Its not ok because then murderers get allowed back on the streets to kill more people, and other criminals are allowed back for their crimes. Also, the police have nothing to do with prosecuting the criminal, the government attorney does, so its not even a prosecutorial abuse of the criminal, therefore it should not be relevant to the prosecution's ability to argue against the criminal.
Although, legally they may have to be let go, it is not right or okay for them to be let go. If there is substantial, and undeniable, evidence that they murdered someone they should go to jail regardless of whether their Miranda rights were read to them or not. This is only right.
I do not believe it is okay for criminals to be let off based on technicalities, this would include murders who are released because someone failed to read them their Miranda Rights. While mistakes are made in the justice system, I do not believe these mistakes justify the full release of a guilty party.