SetDocument runs every time you switch the document

If sizzle switches between parsing a dom document and an xml document, it calls SetDocument every time it switches. Do you think it would make sense to cache more than just the current document settings?

I created a quick jsfiddle here http://jsfiddle.net/5j9xxocs/ . If you run a profile and then click on the "ClickMe" button, you can see that Sizzle's SetDocument is pretty expensive and runs twice for every iteration. I work on an application that uses Sizzle and switches between parsing XML and DOM frequently. I made a rudimentary fix on my local version of Sizzle that caches the settings for both document types and was able to pretty massively improve the performance of the app.

Would this be something worth adding to Sizzle? Or is my use case pretty edge?