Ok, got your message, now let us have xsl re-use!
From the Microsoft Knowledgebase -
The <xsl:import> element is used to import an external XSLT file. The <xsl:include> element is used to include an external XSLT file. You cannot use these elements in custom XSLT files that are defined in the Custom XSL Path Grid Properties in a BizTalk project. You cannot do this because a Uniform Resource Identifier (URI) that is defined in an external XSLT file may be from a nonsecure source such as the Internet.
Am I (well, and Ben Gimblett here) the only one who thinks this is a lame excuse? since when MS tries to protect developers from stupidity? and in any case, if they really wanted to do that – wouldn’t they have to prevent us from writing ANY code?