Personally, I've always this strong conviction that I'm not talking to a photographer but a equipment collector who ran out of reasons to give when they start talking about build.
Other than working photojournalists (they throw their lenses around, drop the lenses, don't cap lenses, etc), I've yet to meet a single photog who talks about build and are actually in situations to make use of it.
I use to own 2 non L EF lenses(Efs 18-55 and EF 28-200)...and they failed on me twice on both lenses. Both lens cannot zoom after dust got into it (still not talking about lens flare). And another occasion, 2 screws drop out from the internal of the 28-200 and Jam my mirror at Hong Kong Disneyland. After those incident, I never dare to trust the non "L". Every since 2 years ago, if I need to shoot on a oversea trip, I will only trust the "Ls".
Recently I went to cambodia, and shot at one location where the dust is so fine that even my 24-105 starts to sweek after a while due to dust. I cannot imagine if I use a non-dust resistant lens in that situation. My face was red(not because I was shy) and my camera turns pinkish and lens filter has to be cleaned constantly to get clear shots.