"This is semantics, but I think the appeal of a 1+ person shelter is that a hiker can enjoy a generous amount of space for just a tiny weight penalty over a 1P shelter, thus increasing comfort on the trail. Most people who've tried to use a 1+ person shelter with their wife can probably tell you that it doesn't work out so well if you're trying to get her to enjoy camping.
Good point about the extra space for minimal weight.
As for spouses, that's how we do it, and it works well for us. Of course, my wife is pretty small, and she prefers the warmth of being so close. (Actually, I'm the one who'd rather have a proper two-person shelter for convenience sake.) Either way, 1+ is a good category.