There has been a thread on mtbr.com's 29"er forum concerning handlebar clamp diameter standards which has prompted me to think about this matter again. Let me first just clear up one thing: 31.8mm bar clamp diameter stems and bars are stupid. I don't like them, and I don't need them. This is my own opinion, and you are welcomed to disagree. I will give you my reasoning though.
You see, it's basic metallurgy/tubing knowledge that tells you that stiffness increases as diameter increases. It's part and parcel to why we are able to ride aluminum bikes especially. Without the alloying processes and tubing diameter, aluminum would just fail as a good material for cycling. This is very simplistic, but for the sake of brevity, please bear with me. Okay, so we have aluminum handlebars and for years they were incredibly lightweight and strong in a 25.4mm diameter for the stem interface. Then some brainiac decided that we could have lighter, stronger aluminum bars if we went to the 31.8mm clamp diameter. Okay, brilliant! That's great for the DH/dirt jumper/Tour de France sprinter crowd, but you also just automatically increased the stiffness of my handlebar exponentially. My hands hate you, by the way!
Look, handlebars are neanderthal technology in the first place,(witness what we have for our other contact points), but that doesn't mean you have to go all medieval on me and torture my poor mits with an unforgiving bar/stem combination. No way will I willingly ride 31.8mm bar and stems on a mountain bike. It hurts for an unnecessary reason. 25.4mm is plenty strong and it's more flexible, which translates into more comfort. Sure, I could use carbon fiber bars, but they are a bit more spendy and don't have the sweep options I like.............yet. I'm hopeful that my favorite bar/stem producing company will rectify that someday soon. Then I'm sure carbon fiber will be served here at Guitar Ted Laboratories. In 25.4mm, of course!