As mentioned above, snails will crawl right over most toothed configurations and most overflow prone fish and shrimp will go right through the teeth
More often than not, the teeth serve no purpose other than eye candy. They are popular because they look nice, not because they perform any critical function.
That said, on some setups with narrow overflows and HIGH flow, they do keep larger fish out of the box. However, in that same case, the surface skimming will be dismal and the entire overflow design should be rethought
In other words, if you NEED teeth to keep the big stuff out, then the overflow is more than a little inadequate for the flow you intend to put over it.