The answer to this depends to some extent on whose sake we are talking about -- in other words is it good for whom that private security officers don't have to do that?
You could argue that it is good for society and the officers that they do not have to Mirandize suspects. Because they do not have to Mirandize the suspects, the suspects may be more likely to confess. This is helpful to the officers and it helps society by making it easier to determine who is guilty of some crime.
On the other hand, if you are talking about the suspects themselves, it is not such a good thing. The suspects might be pressured into making statements that would incriminate themselves. This is, presumably, not good for them.