Depends on what you mean by "go bad". They are hydraulic motors with some valving in them. Generally speaking I wouldn't expect the motors to go bad unless something else in the system goes bad first (aka you shred a pump and put a bunch of metal into the system. What I also learned is that the steering relief valve is built into the pump, and I had the bad luck of having my valve stick open. I tried taking out the shuttle and cleaning it up, but it just stuck again.
So long story short, yes, they can go bad, though I think actually using them will probably keep them alive longer than just letting them sit. I'd be more worried about a valve gumming up or getting corrosion on it from sitting around in one spot than having constant fluid flow. Ultimately they are just a casting that's full of parts, so unless the casting itself gets really beat up or corroded, I would bet someone out there could probably reman the guts. Hard to say how much that might cost as it might take a machine shop making a one-off piece to fix it, but my point of view has always been "someone made this part before, so someone can make it again if needed".
