There were people that had issues the way that I did it, but things were so expensive that I felt like I was doing the right thing.
With my friends that had significant others, I invited them both. To single friends, I just invited them. Because, hey, the singles table is where the fun is and anyways, you want to be unattached so you can flirt with the bridesmaids/groomsmen, right?
My husband chose to invite a couple of his coworkers, but that was his decision. I didn't invite mine. One thing that got me a bit upset was getting a response card from a single person that said "So-and-so plus guest" with a notation that he'd find someone to bring closer to the actual date of the wedding. Ummmm... I just thought that was kind of presumptuous, since the invite wasn't addressed to "and guest", but I just assumed he wasn't sure of what to do because he was a guy?
Since our wedding was here and 95% of the people were coming in from out-of-town, we invited everyone to both. But I agree, that I've seen it where just family and close friends are invited to the ceremony, and then everyone else joins at the reception.