So long as you book your just-in-case room at a property that can more than handle that kind of booking, I'd have zero qualms whatsoever about doing so. I'll recycle what I posted in another thread asking this same thing:
I don't necessarily think there's anything wrong with booking a room
because you're honestly intending to use it if the situation arises
where you must. There's nothing underhanded about that, and in fact
hotels often have it built in to their pricing structure some folks
are less committed and less certain to actually stay than others.
Mind you, if it's a small B&B or a little boutique hotel with very
limited rooms that's one thing. But if you're booking your backup room
at a large chain (especially something like, say, a Residence Inn or
other such place with a kitchenette and longer stays in mind) their
rates all interact with one another. People with flexible bookings are
on the hook for a higher rate if and when they show up; folks who've
committed with a prepaid reservation lose out on flexibility but make
up for that with savings; some large hotel chains will even adjust the
inventory that is shown with all this in mind (only a certain number
of a particular room type can be booked on a flexible rate, while
reserving the rest for prepaid; no package deals or significant
discounts on a particular room type because they never have a problem
charging full flexible rates for it; flexible rates take in to account
a certain percentage of those guests will cancel, but they more than
make up for it with the inflated prices the other flexible reservation
guests pay, on top of the money already in hand from prepaid
reservations).
You as the consumer gamble, as does the hotel - if you end up having
to stay and pay a flexible rate to do so, the hotel is probably going
to make tons more off you than they would many of their other guests;
if you don't need the room and have to cancel, and demand is so high
and inventory is so low your booking put them so close to being full,
the last minute rate they can slap on your newly available room would
be pure gravy for them if someone booked it. If your booking put them
so close to being full that it really did push other potential guests
out, they're probably doing just fine. In fact, many hotels don't even
shoot for 100% occupancy - if your hotel is full all the time, it
means your rates are too low. Occupancy of around 80% if often
preferred, because it means they're charging so much they can't sell
every room but all the people who are staying there are paying high
enough rates for the hotel to profit the most.
If you truly have every intention of staying at that hotel if and when
the need to do so arises - even if it's not certain it ever will -
yours is just one among the many flexible reservation bookings they'll
have at any given moment and deal with all the time.
Now if you're finding some super discounted flexible rate through some
third party booking engine, maybe that would put the hotel out if you
canceled. Or if you're worried about how much more a flexible rate is,
that's valid. If you really want a clear conscience, book the higher
flexible rate at a hotel, while also booking yourself some trip delay
insurance. That way, if you are stuck having to pay those higher
flexible rates (and the hotel won out on selling a room at inflated
prices to someone), at least you can pass the bill on to your
insurance company - who has it built in to the premiums all their
customers paid a tiny percentage will file a claim, while the rest are
pure profit.