Whenever I draw up even a relatively simple board I always pour a ground plane on both sides. This makes things easy from the point of view that I don’t have to be overly concerned about how to get a ground track to everywhere it has to go. Also makes for great low impedance earth run. I am very aware of the significance of ground loops and coupling through shared earth runs. Thirdly, it doesn’t consume so much etchant because only relatively little copper has to be removed. Why waste it for no good reason, even if someone else is doing the board manufacture? 
Anyway, is there a downside to having a ground plane if it is not absolutely necessary? Any stuff with a decent bit of voltage I raise above the board, and am aware of the extra heat needed to solder & unsolder some connections.