Parsing XML with anything but

Beautiful Soup, which I first learnt of his on his website seems to be quite widely popular. In fact that rolling your own XML parser seems de-rigeur for each language community, even amongst communities that ought to know better (what terror could XSLT pose for a Haskell dev?) 

So why are people so determined to parse (XHT)ML with anything but the tools that God (....sorry I meant Dr K and co) blessed us with and what do they gain or lose from doing so.