"what is the justification for not including them as a matter of course?"
- there are numerous choices when it comes to type and length of hose. Cheaper rubber hoses, more expensive stainless steel braided jacket rubber hoses, lengths from 4' to 12' long, etc.
"Can old fill hoses be reused? If not, why not?"
- They certainly can be reused. If you are confident enough that they are in good shape then feel free to use old hoses (you should replace the washers at the end connections).
The downside is that, unless you turn your water supply valves off every time you leave your house, an old hose that fails can easily cause catastrophic flooding of your home. You need to make your decision based on a risk assessment putting the cost of new high quality hoses against the chances of and cost of a hose failure.
I bought brand new supply valves with quick 1/4 turn ball-valve construction to facilitate easy shut-off, and the best quality braided stainless hoses; and I shut off the valves whenever I will be gone for more than a day, because I do not want to come home to a flooded house.