As I recall it lowers the chances both of getting a usable center *and* the average quality of the result.
No, it doesn't. That would be stupid.
All it does is this:
If you try for any center whatsoever, you get a random center (not all types are equally likely) almost always. The chance of complete failure is very low.
If you try for a specific center, you have a better chance of getting that type, but if you don't, then you get a useless center.
