I am relatively conservative, have been diving over 30 years and -- like others who have posted on this thread -- always do a buddy check. It does not matter whether I am diving with an instabuddy or one of my regular diving crew. I also do a bubble check on every dive. I do this partially out of courtesy, partially out of self defense and partially because -- even though I have been diving for decades -- I am not perfect.
Whether they do a buddy check or not, every person who says that "you need to take full responsibility for your safety/gear" is absolutely right.... You should take care of your gear, assemble it and put it on as if you are diving by yourself every single time... And, if you do, it will be a rare occassion that your buddy finds anything wrong.
However, everyone who answers that they do a buddy check to ensure both the safety of their buddy and that their buddy does not put them in undue danger is correct too. While diving is incredibly safe, it is not uncommon to read or hear about a diver seriously injured/killed either because they were trying to rescue another diver or because a diver with a gear failure (or out of air) panicked and overwhelmed their buddy. A preventable gear failure corrected in the buddy check is potentially two lives saved... Not just one. More than once I have refused to be paired with an instabuddy because the buddy check revealed their gear was held together with duct tape and bubble gum... It is just not worth the risk. On a much less onerous note, if my buddy has to shorten their dive because of a problem that could have been caught/fixed on the boat, my dive gets shortened too... I am selfish... I want every minute in the water possible!
The two other benefits of the buddy check that come to mind are:
1. Making sure you and your buddy know where each other's BC fasteners and weight releases are (essential for rescue).
2. It provides a quiet transition from the comradery and irreverance of the boat ride to getting down to business for the dive. The buddy check for me is the time where we settle down, break the larger group into pairs and acknowledge it is time to get serious.
Throughout your years of diving you will encounter a wide range of abilities, philosophies and personalities in your diving... When it comes to your personal safety and enjoyment, the most important things are: 1) Surround yourself with divers of like mind, 2) Take responsibility for your own safety, 3) Do what you believe is right and don't worry about what anyone else does or says... Unless, of course, you both respect them and after careful consideration believe they are right.
P