The relative voltage between hot and neutral is constantly reversing but that doesn't mean that the two conductors are the same. Relative to earth the hot is varying (both positive and negative) while the neutral stays at roughly zero voltage relative to earth.
In particular if a single pole switching or protection device ends up in the neutral due to reverse polarity it can leave the appliance in an "off but live" state which is undesirable. A single pole protective device that ends up in the neutral also can't provide any protection against faults to earth.
It is possible to design applicances that meet safety requirements even if live and neutral are reversed and most modern appliances are likely to be built this way since in many parts of the world unpolarised plugs are the norm.
However if you live in a country where correct polarity is expected (such as the US and the UK) you should respect that expectation to ensure that all equipment is safe, even older equipment or equipment that was built in your country and never intended to be exported.