^ @
cyccommute , I enjoy your thoughtful posts. I know that you're passionate about riding all sorts of bikes. You're also a stalwart contributor to BF discussions. But I think that your response has gone off track from the OP somewhat. He just wanted to know "why" and comes at this from a comfort/hybrid road perspective (2018 Giant Sedona). He doesn't tour, and he doesn't ride off road. I've done both, so get where you're coming from.
I, too, have had many triples, but am coming to the conclusion that they're really not necessary in most cases. I ride with a friend who is quite an accomplished cross country MTB-er, very technically sound and aggressive. He says that the XC MTB scene has (past tense) moved away from triples to doubles, and now is settling on wider range singles even. This may not apply to really mountainous riding. I just don't know. But those would be the extreme cases anyway.
For touring, I get it. I also appreciate the convenience of having a 0.75 or below gear for late in the day, loaded, and climbing. And you need the big ring for downhills and tailwindy days.
But for the majority of the recreational riding and all of the competitive riding, a triple doesn't make sense. You can get useful gear ranges for any skill level with the proper selection of double chainrings and the correct range of cogs.
I'm surprised that there has been pushback on my assertion that triple front/rear shifting is clumsy. There is more overlap, so finding the "next" adjacent gear takes more mental effort. The middle position is rarely as crisp as a limit shift, and there are three center trim positions. I've shifted with STIs, bar ends, triggers and grips, and I don't find the triple shifting as sure or silent as with the best STIs with a double. It's really not even close.
Finally, any weight you don't need is just...weight. I was commuting on my fast road bike yesterday morning, and some fellow passed me going uphill on a somewhat extended 7 degree incline. So, reacting, I began to apply power. I wasn't catching him and was astonished that I couldn't. Shocked. Minutes later, I arrived at work and took off my backpack, which I'd stuffed with food, lunch, laptop and other sundry items. 25 pounds, give or take. A headslap moment...right there.