>   I'd say it was to avoid some trouble.  I did make the mistake a few
> times: importing the schema is intuitive, but I got a few 
> times wrong results, then understood that I forgot to 
> validate the input while invoquing the transformation...

Yes, it's a very easy mistake to make.

I tried to get a change into the spec which I called "assumed validity", so
that a stylesheet that imported a schema should implicitly assert that input
documents conformed to (and were validated against) the imported schema - or
possibly even trigger the validation. But I didn't manage to articulate it
clearly enough to convince my colleagues.

Michael Kay
http://www.saxonica.com/