A bad idea IMHO, and not very useful in practice. They basically put the power from the 30A on one leg of the 50A line and the power from the 15A on the other leg of the 50A line.
Why is this a bad idea? Let me enumerate some of the ways...
- As was said, this will not work with a GFCI protected 20A (or 30A) receptacle, such as has been required by the NEC for camp sites for quite some time. The number of places where these are usable is rather slim.
- In the event of a faulty neutral connection on the 30A side, you could end up with 45A going down the 15A's neutral line. That's a significant overload and fire hazard.
- With miswired outlets, you could end up with live hot wires on one of the plugs when connecting or disconnecting.
- With miswired outlets, you can create a hard short when plugging in. Hopefully this will trip the circuit breaker.
- At least in some cases, the 15/20A socket "shares" the circuit with the 30A socket, so you don't actually get any additional power available. (The 30A breaker would be a master breaker for the pedestal in such a setup.) I don't know that this is a super common setup.