For export, only really when sharing files with users of PT9 and earlier. For import, there are lots of scenarios where I want interleaved instead of split stereo. Eg prepping batches of files for use in Kontakt, where it’s much more efficient to work that way, and simply panning channels leaves you with a lot of bouncing/rendering to do.
Anyway, the post is kinda redundant now, as since posting this I’ve discovered Suite Spot Studios Cleaver, a donationware utility that can join any two mono files to stereo interleaved, and can split interleaved to dual mono. Windows only but I’ve had it running on OSX Mavericks under Wine, so if you have need of this sort of facility I can recommend it. Pairing AA Translator with Reaper plus SWS extensions also does a fine job.