long term reliability and optical performance are the main reasons i will not buy 3rd party lenses (excluding zeiss though i only own that brand for my hassi).
even with the hype of matching or exceeding performance, these new 3rd party lenses haven't been tested for durability or reliance. only time in the field will tell if they hold up. my L glass has performed exceptionally well over a long period of time in some rugged conditions. i have heard far too many stories of Tamron's or Sigma's focus mechanisms failing or lens creep setting in etc. historically they just haven't been built as well as top of the line canon glass.
No, but then a lot of them cost a fraction of the price and not everyone needs to cope with rugged conditions. As for the superiority of "the real thing", the autofocus mechanism on my first DSLR (a Nikon D3100) completely died after four months, the Pentax K-5 I later replaced it with arrived with the famous stained sensor problem that the first batch had, the 5DII with which I eventually replaced that arrived (brand new) with a dirty sensor), my Canon S95 developed a peculiar flaw after four months, and my 70-300L died a week ago (stopped focusing at all, manually or otherwise, and the zoom more-or-less seized up). All these problems were promptly and efficiently fixed, but still....
As for resale value, I guess it depends on the lens. When I recently sold all my Pentax equipment, aside from the K-5 body I got more-or-less what I paid for it all (more, in the case of some lenses I had bought second-hand), including a Sigma 8-16mm, a Tamron 17-50 f/2.8 and Tamron 70-200 f/2.8.
Now I'm in the Canon camp, it's true that almost all my lenses are Canon, but I gladly bought a Sigma 50-500 OS recently and doubt I'll wait much longer before buying a Sigma 85mm 1.4 (I rented one a while back and loved it). If a lens does what I want it to do, and I can afford it, I'll buy it, regardless of who makes it, and if I end up losing money selling it, well, if it did what I wanted it to do, then it was worth it.