1) Many shoes (and most jeans too) don't fit me in whatever size — either the fit is more complicated than a simple scalar, and/or sometimes I'd need a size somewhere between two of the sizes offered.
2) Returns on the scale needed are thus generally a bit of a pain.
3) I suspect that the overhead of paying for lots of returns makes these services rather dear, and since I find jeans and shoes way overpriced anyway this puts me off. (This one is probably all in my head, but it's no less real for that).
What I'd really like is a service where I could identify to it some clothes/shoes that fit me really well, and it could offer me a wide range of other clothes/shoes with a very similar fit. And isn't outrageously dear. (By all means, tell me this exists!).
When I get home later I will try to remember to give this a shot. The concept is awesome and it really is a "value add" worth some markup if you can shop online with some realistic expectation of fit.
And I understand the issue with less expensive stuff. The tolerances on cheap clothing (and plenty of not so cheap clothing) must make it hard. I've bought multiple pairs of the same "number" Levis and they fit completely differently due to how they cut the fabric I guess.
So many places have great return policies, so it doesn't really matter anymore …. http://www.zappos.com/ is a good example.
(Another example of a fantastic return policy, in a rather different domain, is REI's (http://www.rei.com/help/return-policy.html). It used to be even better—lifetime, or maybe just 10 years?—but my understanding is that people would take ridiculous advantage of it.)