Just because you've never experienced something, does not mean it doesn't happen. Because I've never gotten into a wreck in a Volvo, I'm not going to make any comments one way or the other about their relative safety in relation to Mercedes or BMW.
You can hook your toes around two spokes at the bottom of the rim, squeeze your knees on both sides of the wheel to keep the front-end stable and yank one bar-end up and push the other down. This is just with my arm strength, imagine an all-out sprint with the muscles from the legs and back being brought into this scenario:
The fact is, there's such a thing known as engineering, using terms like modulus-of-elasticity and equations that depict the polar moment-of-inertia (radial stiffness) of tubing. For any given design and material, you can calculate the amount of deflection based upon the loads (or just pull out a ruler and measure the distance from each bar-end to the fork-crown in those photos). There's a reason bikes aren't made from 3/4" diameter tubing and why softer materials like titanium or aluminium are made in larger diameters than steel.
In the 10-years I've worked at a shop, I must've seen over 50 snapped stems, all of them quill stems, never a threadless. Although I have seen a few threadless stems with stripped-out clamp bolts. I've snapped two quill-stems in finishing sprints at races and an SR handlebar too. Refer to Kirichenko's 1989 World Championship event... and you'll know why track guys use steel handlebars and stems...
There's a difference between
facts and
rationalization. It doesn't matter
WHY something is designed or
WHY it's used, you can come up with an infinite numbers of reasons on both sides. Here are just the general facts, regardless of "why":
QUILL STEMS
- heavier
- more flexible
- easier to twist in fork
- easier to adjust for height
- one-piece wrapping clamp tends to be quieter, although tougher to change bars
- looks cooler when horizontal without too much of a handlebar-drop
THREADLESS STEMS
- lighter
- stiffer
- harder to twist on fork (due to larger clamping area)
- tough to adjust for height without future planning during installatoin
- might not look as cool when adjusted for comfortable handlebar-drop
- easier to change bars, but 4-bolt clamp can squeak
- pretty much the only way to install a stem on an aluminium or CF steerer tube
There are pros and cons to both, you just have to pick a list of criteria that's important to you and find the system that best fits those criteria.
Yes, you can adjust them with the same amount of precision as threaded headsets. They also come in both ball-bearing and roller-bearing versions. I like the roller-bearings since they can withstand more load and lasts longer. Notice that on my bike above, I have the roller-bearing Stronglight threaded headset (62gm) and I've left enough extra steerer tube to use with a threadless stem if I wanted to...