> take for example xsl:sort
> 
> <xsl:sort select="." data-type="number"/>
> 
> what does 'number' mean here?

It's retained for backwards compatibility with XSLT 1.0; the "native" way of
doing this in 2.0 would be

<xsl:sort select="xs:double(.)"/>

if they are doubles, or more likely

<xsl:sort select="xs:integer(.)"/>

if they are integers.

> 
> and if we are comfortable with this type of ambiguity of identifying
> something purely as a 'number'....then why not have some fairly useful
> functions such as is-number() and is-string() that exist in 
> this no mans land.

You're welcome to write your own function is-string($x) that has the same
effect as the expression ($x instance of xs:string), but the WG wants to
avoid bloating the language with redundant ways of doing the same thing.

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