What are the use cases for having tunnel="yes" on xsl:params and variables?

It must be in the spec for some good reason, though I hestisate using the
feature
fearing that the use thereof may lead to a poor programming practice.

I well remember the list's attitude to local functions had these been
allowed in XSLT.

Thanks for the insight,
Justin Johansson

*** A horse with no name is called Lambda ***