Hey everybody,
Here's a few things to consider in this discussion: Gravity and the resulting acceleration therefrom is the thing that takes our energy when we're pedaling like mad or just cruising. Conservation of energy pretty much says that a body at rest tends to stay at rest and a body in motion tends to stay in motion. It requires energy to change those states and, obviously, the more mass an object has the more energy is required to change that state. These Newtonian physical laws are inescapable unless we're very, very far away from the planet or dead, which of course cancels out the enjoyment of a bicycle ride of any kind.
So, once you accelerate to whatever speed you want to ride, you want to stay at that speed at all costs, especially if you're riding an individual time trial or Tri. There's where all the aero comes in real handy and the extra weight can actually help you. When you're full tilt boogie against the clock, aero saves you precious grams of energy reserves for the final kick. You're trying to stay at as fast a constant speed as you can. It's just the rider, the bike and the wind...and what you had for breakfast. The extra mass of the aero wheels helps a bit to keep you at your chosen speed and the aero helps a lot to cut that wind resistance. Just hope you don't that strong cross-wind that could potentially take you off the bike. Ever watch the TDF individual or team time trial? Aerodynamic poetry in motion for sure!! It's all about aero, conservation of energy and 100ths of seconds!
Head out on the road in the peloton or by yourself and you have a whole different ride. Same physics apply, but the needs in applying those physics are very different. As pointed out by others, one needs to be able to accelerate quickly and, sometimes often, especially for that final sprint to the line. Less mass is better for the same conservation of energy reasons. You use less energy accelerating less mass. Then when the road turns up, gravity really takes over. The steeper the climb, the harder you have to work because gravity is trying to make you fall back down the hill. I don't need the vector math to explain that or what you know will happen when you go over the top and start to descend. Cross winds are your enemy and your enemy takes you out with cross sectional area. Cross winds can be almost as bad going up, although not as deadly. If you're going up a mountain and you climb say 1200 m in less than 10 km, wouldn't you want to be as light as possible to maximize your energy use? Add to that the fact that you will be repeating that "little" climb 3 or 5 more times before the end of the ride and you have everyone in the group wanting to be first over the top.
Competitive cyclists want to maximize everything for the race. Do you see TDF, Vuelta, Giro riders riding aero anywhere but the time trials? No, because you need stability, agility, comfort, response and strength for all other race conditions. Imagine being in a tight group at 30mph+ and you're feeling it with your aero wheels. Suddenly gusty cross winds come up, you get pushed to your left or right and take out whatever part of the group is right behind you. How about the cobbles or other really rough road? You won't get far on those incredibly stiff wheels. They'll break or you'll break and your done.
As others have said, drafting is the way to conserve energy in the group. Aero is the way to go if you're racing the clock. Otherwise, it's all about the coolness factor and how much money you spent or saved.
Anyway, it's sunny and mild in L.A., so I think I'll go ride!!!
Cheers!