Writing valid XHTML was good practice for writing valid XSLT. Browsers would definitely choke on invalid XSL. I remember spending a fair amount of time looking at that pale yellow Firefox error screen that would point out invalid XML documents during my XSL days.
It's been a while, so I'm not completely sure this is true, but I think any XHTML within an XSL document also had to be valid XML (that would also validate against the XHTML schema), so you couldn't get away with as much as you could with rendering a plain XHTML page.
It's been a while, so I'm not completely sure this is true, but I think any XHTML within an XSL document also had to be valid XML (that would also validate against the XHTML schema), so you couldn't get away with as much as you could with rendering a plain XHTML page.