I would love this. We use "Always check out a clean copy" for a nightly build, and for that build this would be very useful. This build don't use the "poll for changes" functionality, so this is not a loss. Also, since this is always a clean copy, the pristine svn-files are absolutely wasteful.
Reading old mailing list threads (such as https://groups.google.com/forum/#!topic/jenkinsci-users/1eN74vwtpcs), there seem to be an understanding that an "export" option is less good than an "update". That is probably true. However, compared to an "Always check out a clean copy", "export" is plain better