How useful/valuable is internal cabling?
One difference that internal cables make is if you transport your bike by car or have to haul it around by hand.


Because the cables are inside the frame, they are less likely to get pinched by the clamp on your car-mount (especially if you use a trunk mount that clamps the top-tube). This is also true for car/bus bike mounts that clamp the down tube.
If you are carrying the bike up flights of steps, having internal cables means your hand/shoulder isn't rubbing the cables when you carry it by the top-tube. 
If you have external cables on your downtube, they can get banged a bit if you lock your bike with a U-lock through the front wheel and triangle. Nothing critical.


These are all really, really minor. Internal cables are more about the aesthetics than anything else.