Even though XSI is superior to Maya in many respects it also falls behind in many others (nurbs, dynamics, soft bodies etc). XSI has it's own host of problems in a real-world production environment. The business model of paying a premium for a supposed "killer 3D app" doesnt hold true anymore, just ask A/W 
Take a look at the forums on highend and the number of posts for each app. Not a scientific analysis by far but go to different forum sites (ie cgtalk) and you see the same pattern. If anything, softimage pricing is preventing a larger market share.
For the price of one XSI advanced license (~$15k), I could get 2 lightwave licenses (~$3k), 2 messiah animate plugins (~$2k), 2 souped up pc's (~$4k), money left over for a tropical island holiday for 2.
It's not the software, it's the artist right?