Well, not going to argue about it, but there are many circumstances where riding side by side may be the safer way to go. It is very difficult to separate a group of bikes riding in close formation, and the bikes can defend each others position very easily in traffic.
I have ridden side by side with a number of people for tens if not hundreds of thousands of miles and never had a problem. It's more a habit then anything else.
When riding side by side with someone you just know casually you have to pay more attention and it may not be worth it, because it increases the fatigue level at the end of the day. It does however make you a better rider because you subconsciously control the position of the bike much more precisely. If you are going to be hanging with someone for a while it might even pay to force them to ride like this as it makes them much more aware of their surroundings.
If there is someone you don't want to ride this way with, tell them.
It is not just a club thing, both DPS here and CHP in CA ride this way too.