When is it a good idea to type a variable/param as "xs:string?" - what
  benefits does it bring over not specifying the "as" attribute?  


if you don't use as then an xsl:variable with content and no select
attribute produces a temporary tree, what we used to lovingly call a
result tree fragment.

<xsl:variable name="x">hello</xsl:variable>

$x is (a sequence of one) document node that has a single text node
child that has string value "hello"


<xsl:variable name="y" as="string?">hello</xsl:variable>

$x is (a sequence of one) string with value "hello"

David

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