First and foremost, I believe in providing everyone with a +1. It's hard going to a wedding single sometimes and having a friend, companion, or SO there always makes it better. That being said, my friend experienced this situation a couple months ago at a wedding she went to for a college friend of hers....
"My friend's wedding was awesome except for the guest that Suzy brought. Diana and James wanted a small wedding and invited only our college group to the ceremony as we've always been close and are the most important people to them (they're not close to their families). Anyway, Suzy was the only single one of the 12 friends invited, so Diana offered her to bring a +1 after Suzy requested it. Well day of the wedding Suzy showed up with some 44 year old guy who refused to take off his sunglasses (her friends are in their mid-twenties). She had apparently met him a few weeks before and they were 'casually dating.' He proceeds to make it generally awkward around all of us throughout the night, even hitting on me in front of my boyfriend. It was well past 10pm when I finally was like, dude, it's long past dark, take off the creepy shades and he said 'No, I'm cool.' Who says that?! Anyway, Diana was pissed. She let Suzy bring a +1 as she said she didn't want to be the only single one there, fine, but then she invited the most creepiest guy ever. Every time we went to take a photograph, HE JUMPED IN THE PICTURE! Even when we stated, hey just taking a picture of the college group only, he was like 'well I'm with Suzy, so I'm in the college group too.' The awkwardness surrounding the photos is insane. All of my photos have him in them, except when I was just with Diana before. Here's the kicker: Suzy just let us know she's not seeing him anymore as 'they were going in two different directions.'
It's scenarios like this...where I can understand my FI's hesitancy on provided +1's to guests where we won't know who they are, or might not know them to well.
PS Yes, I changed the names.