Re: use-when attribute?

I was originally using keys (at your suggestion, in fact), but Mike Kay at one point seemed to suggest it better for my purposes to use a variable. I didn't quite understand why, but here's his explanation:

====
I don't really see what a key with a constant "use" expression achieves.
Apart from the fact that it applies to whichever document is current at the
time, you could just as well (and probably better) use a global variable

<xsl:variable name="citekey" select="//db:biblioref/@linkend"/>
====

You said:
.... but I'm worried that's a bad idea from a performance standpoint. Any other alternatives?