XSI Foundation is not a demo or crippled version of XSI, it's a full application. The difference between the three is explained on the softimage web site.
Many companies will buy a combination of the three XSI versions and use them together.
There was already a demo or learning edition of XSI, it's called "XSI Experience".
In my opinion, XSI Foundation does generally more than the 'full' version of products like Lightwave. Foundation has the SubD modeling+animation mixing+particles+mental ray+scripting of the other two XSI versions.