I'll accept what you say about the build. As to whether in practice the old lens is fragile, it is probably more than good enough for most users. But, I won't argue your points.
It's mostly the focusing ring that feels flimsy and fast, rather than sturdy and tight. Kind of like the difference between a "power steering" wheel on a car versus a tight non-power-steering feel ...
Ah, no I didn't say the new lens is no more than marketing. It is quite the opposite. As you say the IS makes it more usable for many. And it dose have better build. My point was that the L designation, whilst having substance, is a clever marketing strategy. It is a way to bring the user's attention to extra features e.g. better build, and to create a sense of something more than the whole. In the same way Canon branded the ASIC in their cameras ie. DIGIC, and lo and behold Canon uses have long discussions about DIGIC. Nikon and others caught on, and we now have Expeed, Bionz and so on.
Gotta give consumers a reason to buy ...
Regarding the image quality there is a widespread opinion among Nikon users that turning on VR reduces image quality albeit ever so slightly, compared to shooting with a tripod. This might be true of the Canon IS. I do not use Canon so I cannot comment. But this is a digression.
I don't think there is anything that will enhance the image quality of a 1:1 shot taken with precise focus on a tripod ... but the issue is more like what Dan Wells said, whereby a hand-held 1:5 shot ... that would have been blown withOUT the image-stabilization (vibration reduction) technology due to hand-shake ... can now be salvaged with it. Surely this has value. Let's face it, how many shots of wildlife can't really wait for exact tripod placement and manipulation beofre the moment is lost? How many shots get blown in the attempt to do so? Fungus and flowers may wait for us, but how about insects and butterflies? In some cases yes; in some cases no. Hand-holding a macro allows for greater flexibility, so new technology allowing for less wasted hand-held attempts and more 'keepers' in trying to capture fleeting situations certainly is worth the extra dinero to somone who really enjoys macro work.
Speaking of which, nice shots on your fungi collection. I have recently taken an interest in this also. Never really thought about it before, but happened to see an interesting specimen a month or so ago, and have since developed quite a fascination for them. I hadn't realized the extent of the diversity of species. I live on 50 acres in Florida and have found at least 40 different species already ... properly-identifying them hasn't been quite so easy though ...
Perception vs. reality. The build quality of the Canon 100 has never been an issue.
Read any threads with someone saying theirs broke ?
Put your hands on a Zeiss 100mm, and turn the focus ring, and then put your hands on the Canon (non-L) and turn the focus ring, and your perception and reality that the Canon feels like a cheap toy by comparison will be the same as mine. Whether the lens breaks all the time is not the issue (mine works extremely well and has never given me anything but sharp images).
I guess it's like comparing the inside of a Toyota Corolla versus the inside of a BMW. Sure the Toyota will get you from Point A to Point B, and the dashboard may never break or rot, but that doesn't change the fact the interior components of the former are made out of cheap plastics and look-n-feel "cheap" ... while the interior components of the latter are made of fine leather and wood and look-n-feel nice.
Doesn't mean the Toyota can't take you to the same places as the BMW can take you ... and last you a long time ... but it will never be as pleasurable a ride. After all, isn't that why Toyota has its own "L"-series also, namely Lexus, for people who enjoy luxury?
Both IS and VR move the lens elements to correct for camera movement. It seems plausible that compared to a totally locked down lens and camera there could be some reduction in image quality.
Exactly. No need to use these features on a totally locked-down tripod shot ... but when creeping around looking for a candid hand-held shot of a rare butterfly in Peru, having the added security of IS/VR might well make the difference between being able to keep your only opportunity of a rare species about to take wing and missing it because of hand-shake ...