And you could choose to import several thousand books into calibre all at once, too -- and flood the temporary directory in the same way. What is your point?
A user who uses a ramdisk for their $TMP has to be careful with or without your plugin. It makes sense to have
one preference, as opposed to unnecessarily complicating the setup instructions.
Or, you could just say
that a user will have to set $TMPDIR if they want to process many files at once. Rather than claiming that it is down to permission issues (which if you use calibre's temporary folder is not a problem, or alternatively it is a problem that breaks calibre in general). Or claiming that it will default to "C:\Program Files\Calibre2\" for no comprehensible reason.
There you go. Flexibility without any cost whatsoever.