Certainly this is changing, what with pro teams and amateurs alike making use of physiologic testing, wind-tunnels, and accurate power data.
The arena of bike fitting has had a few stabs at this, but many (like Specialized's BG Fit) are tainted by a corporate and retail driven focus.
So I guess fads and marketing need to be treated with some apprehension.
In the realm of physical therapy and sports rehabilitation, certainly there are fads and marketing within the industry, but good therapists tend to use what works -- which is, most often, sound exercise regimens and manual treatment techniques -- not the Tony Little Gazelle, the Ab-Lounger, or that electrical stimulation belt for 6-pack abs.
For this reason, I tend to default to good old published research, whenever I wish a fresh angle on some idea. It's more work - information isn't already broken up into sound-bite worthy tidbits by some marketing department, it's not immune to corporate influence (many studies ARE funded by corporations with an angle to support), and some studies are just not set up very well, so they may or may not really tell us anything with any degree of certainty. But that is why getting good information from them is more satisfying - because it does take a little work and you have to be discerning in your reading.
So as often as I can, I will share some of the more interesting things that are out there -- I think many will be surprised (I know I am constantly) at what some of the research shows. Here's a taste:
Did you know that a study was done about 3 years ago looking at the most efficient crank length for trained cyclists? They tested riders with crank lengths varying from 130mm up to 220mm, and found no significant difference for even some of the most extreme differences. Granted the test was a very short and intense (I believe it may have been as brief as 3 or 4 minutes) but the fact that a cyclist could score anywhere close with 130mm cranks as they did with 200mm cranks, on any test, is amazing. It certainly puts into perspective how futile the hand-wringing regarding 175 vs 172.5 cranks, that many cyclists do, may be.