If you damage a road wheel/tyre, of course a dealer will be able to get a matching replacement at a price, assuming your car isn't historic, so there's no question of suitable wheels/tyres not being available.
Because I routinely fit larger size tyres at replacement time I inevitably end up with a different sized spare which I have to treat as a spacesaver, ie emergency use only. I don't regard this as a problem - if I have to use this differently sized spare while towing, I'll simply drive at restricted speed to the nearest repairer and get the other tyre repaired or replaced.
For those that won't tolerate the time delay that this might cause, a full-size spare is their only option.
Perhaps Nissan should be asked a different question - "Is the spacesaver suitable for travelling to a repairer at restricted speed, after a puncture, even when towing?"
Asking simply whether you can tow with a spacesaver fitted is asking for approval to drive hundreds of miles at 60mph - of course they're unlikely to give their approval.