Of course, they look pretty. But what I do not understand is: How will having taller rims or composite wheels improve a ride? Is the increase in efficiency drastic enough to make you think twice while changing rims? Or is it really not that necessary with the cash it demands, for a tiny bit of improvement?
PS: I tried to find out about this on the internet, but I have found bulky articles about them, so frankly I didn't read them and decided to ask here.
Given graphs like the first one on this page, which show how steeply wind resistance increases with speed, I take it that small changes in shape can be relatively important/effective at high speeds (and not at relatively low speeds).
E.g. my top speed when commuting is about 30 kph; the tour de France speeds on the flat are more like 50 kph. The power required to overcome wind resistance at 50 kph is more than triple that required at 30 kph.
It's also relatively important/effective if and only if you've already improved/maximised everything else (e.g. the rider's position and clothing).
Here's the skinny from my perspective.
Two factors: weight & aerodynamics
Carbon wheels are super lightweight which requires less power to spin.
Taller(Deep Profile) rims are more aerodynamic, so once moving they reduce wind resistance. Obviously, the taller the rim, the more material, therefore they are also heavier. Many folks talk about spin up, which means how quickly one is able to get the bike up to speed, when discussing Taller/Deep Profile wheels.
The ideal wheel is a balance between these two factors, but then again, it really boils down to your goal? Are you racing? If so, what is type of racing? Are you able to afford/maintain multiple sets of wheels?
This is a great question because it deals with the one area that many inexperienced riders underestimate. Wheels IMO, after the frameset, is the most important component of speed, weight and ride quality!
Please keep in mind this is only scratching the surface, but nonetheless I hope it helps!
The benefits of an "aero" bike over a standard bike are minimal and likely of little use save in racing. Over a time-trial of perhaps 30k, Someone with an entire aero package (bike, skinsuit, booties, helmet...) might gain only a few seconds compared to riding his usual bike and gear. But that can be the margin of victory.
There is a reasonable summary of bicycle aerodynamic improvements on Sheldon's page. It's only 1000 words or so.
Broadly, wheels don't make a huge difference on their own. A few percent at most. If you want a dramatic change you have to make a dramatic change - my velomobile goes at least 30% faster for the same power input than any of my other bikes but it looks nothing like them and is unlikely ever to be approved by the UCI. With handicap bikes(pdf) it's all about incremental improvements adding up to a significant overall change, with political lobbying and bribery to get the rules changed to allow further improvements. Which is why you don't see those wheels on the Tour - they're not allowed.
The advantage of disk/low spoke count wheels in this context is that they're easy to use. You just leave them on your bike and they work. A skinsuit is annoying to put on, restrictive to wear and expensive to replace. For about the same aerodynamic benefit.