One, I disagree with you about shimano groupsets. At first, the lower end groups do shift fine, but it has been my experience that as the year goes on the higher end groups work better longer. This has been my experience. For the money difference between a 105 group and an Ultegra group, I'd get (and did get) the Ultegra group because of my past experience with Shimano groupsets. We could argue about this for days and never get anywhere, but it is IMHO.
I ride with an older lady who bought a project one Trek Madone (now at least 5 years old) and spent considerable money to get it light, as light as she could. It's a very, very light bike; carbon everywhere with Sram Red. The reason she did that is to use the bike in her advancing years so it would be easier to get up hills and go a bit faster for her. I'm sure it works for her...she is still riding and happy, which is what this is all about anyway. It's totally worth it for her to spend the scratch on a lighter, "faster," better performing bike. She is certainly not a racer, nor has any aspirations to enter a race but is still a happy "high end" race bike customer.
I'm not a carbon fan, so I'm not going to go out defending the material...but it seems to make fine bikes and wheels. I went aluminum for my wheels and I stop fine.
I agree...not worth it to me either.
For some it is though.
My previous road bike had 105 (purchased in the 90's). I rode it for a decade. 2 years with significant mileage and then after used as a commuter/utility bike. I never replaced the chain, rarely lubed it, and it always shifted well.
My mtb: deore front and rear mech. A bit better on maintenance, lighter use in mileage:
10+ years of use, nothing replaced, still shifts perfectly.
Too early to say on my current tiagra 10. Shifts perfectly though. I don't anticipate any deterioration. 1,2, and a couple of 3 tooth jumps provide almost no challenge for a rear mech. Shimano just works.
Low end v brakes: ridiculous stopping power.
Low endish tektro: average stopping power, but breaking in very well. Kool stops as backups just in case.
I understand people wanting the latest toys, bragging rights, ego gratification, etc. But entry level shimano tech works so well, I simply don't see the need to step up to high end stuff.
I compared specialized's alu offerings (secteur and allez smartweld) with their carbon bikes, and I felt the carbons were relatively stiff and their alu frames didn't give up much, if anything, in terms of comfort. I went alu, and saved myself the hassle of dealing with carbon. Not to mention many hundreds of dollars.
Extremely satisfied with my purchase. I can save 1.5 lbs. off the bike with $350 pair of wheels, and upgrade the brakes to get excellent stopping power with kool stops. I've already invested about $12 in the pads, an incredible value. My bike is a hair over 20 lbs. with clipless pedals, and I can get that down to 18.5 lbs very easily, at around $1600 total. Not to mention I'll have two pairs of wheels with my 18.5 lb bike.