The reason someone might want to use a fly rod with a spoon is line control. Using a 15' sink tip line a fisherman can mend the line to keep the spoon in the strike zone longer and can control the speed of the drift much better.

As I said before I have used a Wells spoon only a few times with a fly rod. I was fishing for silvers on the Skagit with a 14' spey rod. The combination of the long rod and floating line allowed for some serious line/lure control which made the whole combination very effictive.