The spring is providing pressure, and holds the wire against the contact area the same way a screw would. The biggest difference is that it provides less pressure; a screw terminal will straighten out a bent wire, while the spring in the stab will merely hold it in place.
If installed properly, there is nothing wrong with a stab and it works fine and dandy and no buildings burn down and everyone is happy. It's if the stab is not installed properly that we see problems- and, unfortunately, it's impossible to inspect and verify the quality of these connections.
My feeling is that the stab is good in theory but unsafe in practice unless very careful care is taken.
