The guests in the OP that brought their own wine glasses are a bit "snobby." I understand that some people have high standards and don't like drinking out of disposable cups, but bringing your own glasses to a BBQ illustrates to everyone present that your standards are higher than theirs.
Personally, I don't like using disposable anything, so I bring my own plastic plate(s) and cup(s) and endure the ribbing good-naturedly. "Yep, I'm a rabid tree-hugger.
That's why I don't use disposable stuff. Did I mention my career in environmental engineering? Well, I don't just talk the talk, I walk the walk, too!"
Now, my take on BYOB to a BBQ. I've also attended Baby's First Birthday Luaus here in the islands. I've been to a (as in "one") BFB Luau where all the food and drink was provided by the hosts. We, as guests, brought a gift for the birthday girl, but no food or beverage. I found out later that some of the hosts' friends and family cooked &/or donated most of the food & beverages served.
Usually, for a BBQ, the host provides some
meat for the grill, some
beverages, and all the utensils and condiments. Without being asked, all the guests brought meat for the grill &/or side dishes for the communal table. Everyone brought what they wanted to drink, but they put it in the host's cooler (host provided the ice). No one needs to say "This party is potluck." Guests always
bring a dish. You aren't considered rude if you don't bring anything; You just aren't acting like a local.
I've been to parties where I only got one glass of wine from the bottle I brought, but that's because people share my taste in wine. Sharing and trying different dishes/beverages is part of the fun. I've heard people say, "Kitty, you might like the apricot beer I brought. It's in the cooler over there."
Not everything has to be shared. DH will only drink Diet Coke. No regular Coke and never Pepsi or generic soda. His Diet Coke stays in the insulated portion of his backpack with the blue ice. My bottle of wine goes on the communal table to be shared. So we do it both ways: Share our BYOB and keep it for our exclusive use. No one has ever commented on it.
carries Diet Coke with him. He doesn't mind being called a "Coke snob." In fact, I think he's proud of it.