It may seem like that, but just because it appears that way doesn't make it so. Somebody has to guard the police station and normally an ordinary policeman would be required to do so, but if it's possible to hire a security guard and put that policeman out on the streets it allows the department to make better use of it's assets and makes it more efficient.

The only way this is not benificial is that it presents a bad image, but it's only bad if you don't know whats going on, it may also be more expensive but if a department can afford to do it, I don't see why they shouldn't.