just a few things that spring to mind:
-Noise. It seems that engine noise is only half of the noise created by an aircraft on approach. The rest is from the airframe, which might increase by 18 wheels whining round.
-Drag, the drag of the wheels will increase in flight with the gear down, probably negligible, but it might cause pitch-down momentum.
-Problems braking and stowing the wheels when retracting.
Otherwise, it sounds like a good idea.