I concur. When hiring with these kinds of small problems, you actually see a lot. First, in my case, it sorts programmers from not-programmers (it's in the 50% ballpark). Then with a problem a bit harder, it helps sort "programmer-in-the-guts" from "programmer-for-a-job". At the end, I've removed 90% of the candidates and I'm left with average to very good programmers.

Moreover, leaving people to do these little tests in the team and proposing them to ask questions whenever they want, plus asking them to explain their solutions helps a lot in seeing the social skill I need (communication, openness...)

Of course, it's not 100% safe, but I'd say it's a good heuristic :-)