The two halfs i did i had a between the bars bottle , aero bottle and on the top tube, with the spare jammed and hidden under the seat. Now that im running clinchers it would be even easier. I think you guys make this too complicated.
There's this thing called a 100 mile training ride, and I don't know about you, but I don't have support stations every 10 miles for that. Also, I carry a phone, possibly keys, credit card/cash. Maybe some glide if something starts chafing. Maps/Directions. And maybe some tools. Oh, and multiple spares.
You see, if you do long course, you have to train long course in the position. Sorry, can't afford/don't want a separate training and racing bike. Sure I troll craigslist for the mythical clone for $1000, but that isn't happening.
The Trek SC showed you can have your cake and eat it too. A stripped down bikes sans accessories, and one with lots of mount points. Yes, it's fun dumping a lot of the training junk for a sprint. But that's a very limited use. I'm not dumping 5k-10k on a couple days a year bike.
If you don't want those options, there are a ton of bikes out there, and there is little to improve on those from a UCI standpoint. However, even the stripped down single bottle "use case" still benefits from an integrated system.
As for behind the seat, it's apparent there's a lot of design that goes into the seat tube and seat post over the last five years. Most people have the dual hanging bottles, but I see that right in the airflow that's channeled by your thighs after it passes around the aero seat tube. That's gonna drag. So is that tire and two air canisters.
What I described provides UCI and non-UCI aerodynamics options, integrated hydration up front, integrated fluid storage for refills, integrated spare kit storage. And some people think that's excessive? Crap, there's people who's entire careers are spent on the ergonomics of phone buttons.
Cervelo, Specialize, and Trek have shown a willingness to put design effort and thorough wind tunnel testing into their bikes. The market has showed interest.
Don't even get me started on Torhans vs Profile keel bottles.