Sure, I get to hold the camera which is very important - but within days, I can hold one from the online retailer as well.
When I bought my current tripod/head, I spent about 1/2 day at my local B&M pro shop...I musta tried 20-30 different combos...with
MY camera and lens.
Are you really sure you tried ALL possible combos?
There are unfortunately too few local stores with the stock on hand to be able to do the kind of variety testing you speak of. Even a big city pro shop may not have Really Right Stuff, Markins, or Arca Swiss heads on hand. Often their only "pro" choices are limited to Manfrotto or Gitzo. I can tell you from experience there is no comparison in ease of use between a custom RRS baseplate vs. the oversized octagonal plate with cork for friction I previously had. How would you be able to know if they don't have them to show you?
Sure, I could have ordered each of these 20-30 combos on-line
I simply narrow down my choices based on manufacturer reputation, maximum/minimum heights, max weights, what others have reported in terms of stability, ease of use, etc. All the data is available. I keep a tape measure handy to get an idea of the physical dimensions. And then I ordered maybe three finalists.
Even with all the in store playing, you still wouldn't know how the tripod feels to carry around in the real world for several days of hiking. Do the legs get hard to tighten when sand gets lodged in the threads? Do levers catch on branches or straps often enough in reality or is that just an unwarranted concern?
it doesn't work for some kinds of products.
Shoes come to my mind as closest to the tripod analogy. I'm quite a fan of Zappos. Here's a product, shoes, that is incredibly fine tuned to an individual fit and a company with the most liberal shipping and return policy around, no cost either way. Yet the concept has proven wildly successful.
Now I DO have my limits. For example, I wouldn't go for one of those mail order brides!
It might work for a dSLR body...
Let's say I have a D3000 and want to move up to a D5100, D90, or D7000. It might be a reasonable thing to order them, one-at-a-time and try them...stop when I'm satisfied. The flip/down side is that if I stop at the D5100, I may never understand how the more pro controls on a D90 would help my photography!
So how would you know you got the best tripod if the store didn't actually carry all the models that are on the market. Not even all the more popular ones?
There is a point when you've reached good enough right?
If there is really a close call, I've been known to buy both (not those specific models) and sent the other camera back. I'm just not that wishy washy on the specifics I need or want in a product. There's no point in finding out how good a D3S with a 70-200 mm lens attached feels in my hand if I have no intention of ever carrying around such a large and heavy assembly. I don't need to see whether I prefer buttons to touch screen displays for example in deciding between a Panasonic or Olympus M-4/3 camera. I know I prefer buttons.
For the more experienced photographer, I think on-line purchasing makes good sense...more so than for the beginner, who usually don't have a clue.
I do learn all I can about it. How else would I know what features are important to me. But that should be standard procedure even when shopping locally.
FULL DISCLOSURE: Before this debate get's out of hand, let it be known that I in fact do support my local camera store - when they carry items that I have an interest in. I would estimate I've done 70% Internet 30% local purchases.