Emily Post says the following:
If they are family or friends, you do not have to invite them with a guest if they are not (1) married, (2) engaged, or (3) living with their significant other.
The only time a bride and groom should feel compelled to invite “and guest” is when you inviting single people you don’t personally know very well. For example: inviting business associates, business associates of your parents, etc.
But you don’t have to for family and friends. And it’s never polite to ask to bring a guest or just bring one when the invitation only lists you. Keep in mind that the bride and groom have a budget to work in and people they wanted to invite are always cut from the list.
I guess one other situation when you would want to give someone the option of bringing a guest is if your wedding was going to be like a formal ball. If you are going to make your all your guests wear a tux and the music is going to be mostly slow, then giving them the option might not be a bad idea (if you are going to have a $50,000+ wedding, then inviting a few guests shouldn’t be a big deal from a budget stand point, but there still could be seating issues.)
Hope this is helpful.