This thread has generated much discussion with many opinions stated as fact. Its likely that many people will read it in the future, and they deserve to see the best possible information. So, not to be argumentive, but to be educational, I submit the following points with more to come later.
1) The very low end of the bicycle scale (for example Target) often have quill stems. To some this indicates that the conversion to threadless stems is not about manufacturing efficiencies. However, its important to understand the global bike economy before esposing such an opinion.
Taiwan is the global center of high-quality bicycle production. This is true because they put a never-ending emphasis on quality, enhanced productivity, and building/supporting this industry. They are not a low labor cost market. As a result, they don't sell much into the Targets of the world. They are a step above. Efficiency for them is about lowering labor costs. The investment in new products and technologies to reduce labor is a sensible step.
China and India follow a different model. They have low labor costs, to manage their overall production costs, they have to curtail capital investment. Hence, keep making what you've been making, and make only the most necessary repairs to existing machinery and processes. These countries specialize in shipping to the Targets of the world.
So, at the bottom of the scale, it is more cost effective to contiue to produce quill stems, even though they entail a higer labor component in the manufacturing process. Once we get beyond the bottom of the scale, it makes sense to curtail the labor component of mfg. and hence we see the adoption of the threadless stem.
NB The comparisions of Taiwan, China, and India are valid, but not quite so Black and White as portrayed here.
The adoption of the threadless stem was driven by manufactuers to rationalize their processes. There are lots of great ideas that haven't been adopted for bikes, and some bad ones that have. The mfgs are in this to make money, not to be your bicycling angels. None of this suggests that one stem type of another is better, or that you the consumer are generally getting shoddy goods. But, only the niave would argue that this or that feature of this or that product must be good, else the mfg wouldn't have done things that way.
Moreover, the threadless stem as it exists today is quite different from the threadless stem as it was first made. weight, extension diameter, bar clamping methods etc. Moreover, when speaking of either a quill stem or a threadless stem, there is significant variation in either category and significant overlap in characteristics between the products in each category. Anyone who disagrees is welcome to provide hard factual evidence to the contrary, but without that, the arguments ring hollow.
We have threadless stems because they rationalize the manufacturing process. This is very similar to the reason for having straight-legged forks. Neither product is bad. Neither product is inhernetly better or worse than its predecessor. Different people like each. But the reason that we have these products is that they have reduced the cost of producing your bicycle.
2) Carbon absorbs shock! Phullease help me. So does steel, titanium, and aluminum. Actually, its the epoxy more than the carbon that absorbs shock. But what difference does that make.
At what frequency does carbon significantly dampen vibration? How much of that frequency do you encounter during your rides? How much does carbon dampen that frequency? I've yet to see a discussion of these factors in any of the popular press or websites. One has to dig into engineering mater to find any answers. So, the "shock absoprtion" advocates are generally working from oft repeated common wisdom and subjective evaluation. Unfortunately those two factors can't be separated in real life as too many experiments to count have proven.
It doesn't matter if Fermilab or NASA can instument a bike a measure differences in dampening. What matters is if these differences are noticable in real world cycling. My experience is that they are not. Moreover, road buzz isn't what gets to me anyway. What I hate are going over frost heaves. 50 or 100 miles of frost heaves are hard on me and hard on the bike. This may not be as bad as cobbles, but if I want something absorbed, this is it.
If you believe that carbon or titanium is more comfortable, great. But without a double blind test, you're not likely to know know the truth value of that claim.
On the other hand, tires do a great job of absorbing shock and buzz. The wider the tire and the lower the pressure, the better job that is done. Naturally, this is counter to many folks thinking: which goes along the lines of the hardest possible tires are necessary for efficiency, and that a shock absorbing frame is therefore required to protect oneself from becomign road-pummled.
Its funny, I can pull up next to another rider on any kind of bike, with any brand of clinchers and wheel combo, and if we assume the same relative position on our bikes and coast, I tie or beat them every time. Why's this funny? Because I'm usually on 700cx28 tires inflated to 90lbs in front and 95lbs in back.
On a smooth surface, high pressures lead to slight improvements in efficiency. But, roads aren't smooth. And too much pressure going over bumps reduces your mechanical efficiency. It also tends to wear out the rider. And if either the rider or the bike can't finish the race, then their speed doesn't matter.
So stiffer is stiffer, but not generally better. Threadless stems have many useful qualities, as do quill stems. Shock absorption comes from tires, not the frame. We have threadless stems because they rationalized the manufacturing process. Threadless stems have gone through much evolution since they were first developed. There are a wide variety of characteristcs among the many brands and models of threadless stems. Its in the individual riders best interest not to get trapped in marketing think, but rather to figure out his/her real cycling needs and then get out for a ride.