[XSLT2.0] Further on use-when on xsl:transform

I also don't understand this:

"This allows all the declarations that depend on the same condition to
be included in one stylesheet module, and for their inclusion or
exclusion to be controlled by a single use-when attribute at the level
of the module." 

Why not use use-when on the xsl:import/include for this? It seems more
straight-forward to my way of thinking.

-- 
Colin Paul Adams
Preston Lancashire