Bad Manners and Brimstone

I agree. I always thought a +1 on an invitation meant that you could bring whomever you wanted to bring.

If in a couple situation then the SO should be named along with address on the envelope.

I agree, though often in conversation about the issue, people will use the term "plus-one" to mean "allowed to bring a date" even if that is restricted to a specific person, just because it's shorter.

but if the name is on the invite, then you don't get to switch (and if you know the person you should put it on).