Actually, I would say that the Miranda decision made police more likely to follow the law, and to respect the rights of the accused. While it is certainly less convenient for them in the course of their normal duties, it increases the amount of time, training and attention supervisors and trainees spend on establishing procedures that recognize a defendant's rights in practice instead of merely on paper.
Since the court has always required proof of guilt beyond a reasonable doubt, I think the impact Miranda warnings have on conviction rates is overstated.
There is a great deal of controversy on this topic. Some people (like the author of the ncpa.org link below) argue that the Miranda decision has "handcuffed" law enforcement. Others argue that the rules set out by this decision have actually helped law enforcement become more professional.
Many, especially conservatives, argue that the decision has hurt law enforcement. They argue that fewer suspects confess now than before the decision was handed down. They argue that this has caused the number of unsolved crimes to increase.
Others argue that the decision actually made police do better work. They argue that the police used to simply rely on confessions, sometimes coerced. Now, it is said, police do a better job of actually collecting evidence against the accused.
The points of view on this issue tend to vary widely, mostly based on ideological grounds.