If you calculate the total gradient of your entire ride, what % do you get?
Your question is a bit vague, because it depends on a) how you calculate it and b) if the ride is a loop.
In the case of a) I can point at a local short climb that is, say, 6 miles long at an average of 6%, but I can get back down the hill with a significantly shorter descent at 17% average. Going down this, while I'm not using the brakes that much, I am certainly not thinking "damnit, if only I had that aero frame! I could be going much faster! gah! life is miserable without the aero frame" That descent, by the way, you don't want to take too fast because it's very windy with off-camber turns. Aero will be the least of my concerns there.
Then, really, to answer your question about a local loop I do 2-3 times mid week depending on my mood, only a short portion of the ride is spent at 0%... I can guess less than 1/4 mile total out of 25 miles, but 15 of those miles are spent above 7% and the rest are either descents or climbing above 3%. See the previously linked chart. Even at 1% gradient gravity's effect jumps up from 1% (at 0% gradient) to 24%!! That's 1% gradient. Take a look at your local roads: even if you live in a flat area, you may
find a lot of your time is spent at a low percentage grade, not 0%. That isn't to say you throw aero out the window, it still matters at those low gradients and relatively high-er speeds (assuming you travel at a high speed or your relative wind speed is significant)... but come on, "weight does not matter?" Please.
Maybe people who live in the Netherlands, where it's 0% the majority of the time, could really use the aero frames above weight. Definitely a strong possibility.
Or if you ride on the track.
It should also be noted - as Tapeworm pointed out over in a thread in the Training forum - that aero gains are really dependant on speed, and the chart calculates 300W consistency... so let's think about that: 300W all out for the entire duration of your ride, regardless of gradient? You better be "motoring" (quoting TP). But, let's go back to reality: it's unlikely you're putting out 300W the entire time, and much more likely that you're going much slower than necessary to get all those aero gains you are seeing from wind tunnel data.
Then again, if you live in a relatively flat area, go for it. There's a reason the market, thankfully, has options and choices, we have diverse riding interests.
That said, my original point stands: weight still matters, gravity is constant, aero gains are variable, the aero frame is quite low on the list of money-spent vs. performance gains.
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