When my stepmom in Ottawa had to be put in a nursing home, the first anyone knew of the water problem was the $5000 waterbill for the unoccupied house. Thanks to all who paid premiums, insurance covered the repairs, and hopefully the trustee learned something about pipes, Canadian winters and the wisdom of reducing the heat to a minimum.
My friend with the Yorkville condo returned from a week away to find a flood from a suite two floors up. The owners had run a wash just before they went on holiday and a bit of solder had kept the solenoid valve from closing so the washer showly refilled, then overflowed for days. Our codes don't require overflow trays for washers, DHW heaters and other such auto-demand/shutoff equiopped appliances. Since the suite between was unoccupied, it was only when the neighbours four floors down noticed that anything was done.
No downside to shutting off the water I can think of, except as noted above: pets and the bathroom-users who might visit by to water them, and hot water heating systems that somehow 'lose' water and are built to demand a feed. But wouldn't you want a human to notice that loss—it likely didn't just evaporate harmlessly— and do something about it?
For lots of other reasons, frequent, regular checks by responsible people are what to aim for. Water's just too fond of going places and doing things we don't like.
