I always have mixed feelings on this.
I understand how difficult it can be to attend a function like a wedding without an escort. Especially if everyone else there is couples. But I don't think every single invitation should include "and guest" for every adult over a certain age.
In these parts, weddings (and bar/bat mitzvahs) are expensive. If you're having an upscale wedding, per person prices can be over $100. To invite each adult with a guest (if that person is not in a long term relationship with the invited guest) could add significant money to your bill, especially if you're inviting a lot of younger people. And then you end up with a lot of people you don't know at your wedding.
When dh and I got married, his brother was not dating anyone. And I mean anyone. But dh said we had to invite him with a guest. Who the heck was he going to invite? He didn't have a girlfriend, wasn't dating anyone. But dh didn't want him sitting alone. Even when he wasn't going to be since he knew all of dh's friends for years and was going to be sitting at a table with them.
So he got to invite a guest. Some girl he knew and dated casually about 5 years before. I didn't know her, never met her, never saw her again. So there was this stranger at my wedding and in my pictures. Did it break the bank? No. Did I like it? No again. If he had been dating someone for a time, I would have been fine with it. But to invite him to bring a stranger to my wedding, I didn't care for.
As for wedding invites, if you live w/ mom and dad, invite goes to their house. If child lives on their own, invite goes to their address. If they are living with someone or dating someone for a significant period of time (even if not engaged), that person gets invited. If they have no special person, then I think they have to deal with either coming alone or they decline. It's certainly their right not to come if they think they are going to be uncomfortable.