Mothra,

I should be out polishing a head-tube, but fair's fair, eh? No I'm not going to dig up minutes of design and engineering meetings at Giant. Nor am I going to research quotes on the subject from BRAIN. Call me lazy if you like.

Instead, let me illustrate the cost savings thusly:
. Henry James stocks 4 lengths of threaded steerers. One can presume that this is a sufficent number of lengths to cover the range of sizes built by a large bicycle mfg. This is four times as many parts to maintain in stock as a single length (long) of threadless steerer. And, to ensure against stock outs, someone (even with a JIT system) needs to maintain buffer stocks which ultimately add to the cost of the finished product.
. The fork mfg, now has to make at least 4 sizes of fork (assuming that only steerer length, not offset) varies among the forks. Once again, there is a need to manage 4 inventories rather than one, and there is a need for 4X the safety stock.
. Threaded steerers need to have the threads applied. Even when this is automated, it is an additional expense which doesn't exist for threadless steerers. And, and automated threading machines aren't exactly free.
. During assembly, the mfg process needs to keep track of which size frame is being built up, and make sure that the correct size of threaded steerer is being assigned to it. This is an additional process and cost, whether its managed proactively, or whether someone grabs a frame, eyeballs it, grabs a fork and holds it up to the frame to confirm that he/she grabbed the right fork.

The cost savings should be obvious, especially related to a large scale MFG.

Some people find their threadless headsets to be more immune from coming out of adjustment, especially off road. I don't doubt this is true, and would represent an advantage for a subset of the larger riding population. I'm not sure that this is the only solution, (heck, I'm not even sure what the real problem cause is). About 2 years ago, I stopped offroad riding. Mostly a time issue, but I also didn't want to ruin my fork.

The last rebuild, I was told that replacement parts probably wouldn't be available to rebuild it again in the future. My bike is an old Stumpjumper, made of Tange Prestige Tig welded in Japan (back when that was still an affordable source of higher-end frames). It has a Future Shock which is essentially a Specialized branded version of the original Rock Shox. It's rather irreplacible, or I would have so done. You see, it's a 1" threaded steerer fork. The threaded headset on this bike (probably a Tange) hasn't had problems of any kind, including no problems maintaining adjustment.

There have been a number of methods for fixing threaded headsets to prevent them from loosing up, but I haven't had a problem in this area, so haven't looked at them too closely. However, of all the reasons that explain why threadless headsets gain a foothold in the market (apart from mfg rationality), preventing headsets from loosening is the most compelling argument. However, the change to threadless headsets is a fairly radical way to resolve the problem of headsets becoming loose. As a consequence, it doesn't explain the rapid and nearly total conversion of the industry to the new "standard".

Remember, threadless stems did not start out as they are today. In fact, they often had extensions that were very similar to those of quill stems - hence they had riding attributes much like quill stems.

For those who remember some of the early Mt Bike equipment, especially at the lower middle price range, there were some odd looking stems and bars produced. Some worked with threaded steerers but had 4 bolt heads. Sometimes bolting from above instead of from in front. There seemed to be a real attempt to affect a pseudo motorcycle look. Bits and pieces of all the various ideas stuck as the threadless stem evolved, and today its quite a competent piece of equipment - provided the headtube on your bike is long enough.

If my arguments aren't compelling to you, well I tried. Just remember, threadless headsets and stems clamped directly to the outside of steerers date back to before WWII. Who knows, maybe the wind will blow in another direction some day and we'll all go gaga, once again, for quill style headsets. In the mean time, enjoy what you have and make sure you put some miles on this and every week. Cuz, not matter what you're riding, the other guy is training hard.