Road riders need lightweight, tiny tools that can fit into jersey pockets. A lot of those items are made of carbon fiber, which is lighter than aluminum. Other iconic trappings of road riding, like CO2 cartridges and spandex outfits, are also geared toward minimalism, but all of that downsizing comes at a cost. Commuters don’t have to be as concerned with weight, so unless you covet something specific, don’t spend the extra money.
First, carbon. You already know you don’t need it for your casual weekend pursuits, because you’re not counting ounces the way a professional road racer would. But to quote an article from Velonews, “Remember that professional athletes operate in an entirely different environment than the rest of us. They are all very close to each other in terms of fitness, and they are also all very close to being the absolute best a human being can be. In short, you’re much better off upgrading your legs and dropping body fat through proper training and diet.”
Modern bottom brackets use a sealed bearing type construction that keeps water and debris locked out, so they seldom go wrong. Maintenance is impossible so they need replacing when they do wear out. Older style cup bottom brackets need to be re-greased every now and again, and occasionally will need to have their bearings replaced. There are a lot more teeth on the chainring cogs than those on the rear cassette, so need changing less regularly. Cranks usually a solid one piece construction so there is not much to go wrong.