I put one in there because the original bean animal plans had one . It's just a ball valve, not a gate. And to be honest I don't really know when or if I'll ever use it.
As for the john guest, can I just drill a hole in the top of the 90 of the open channel? To do this I assume the water level would need to be below the hole and the hole would need to be below the emergency drain height (which as mentioned above I think I need to lower a tad).