This method name is really a misnomer and we should really fix it, but scope creep, so not now.  But let's fix this comment to be something like:

Given a list of messages, download snippets for those that don't already have snippets.  You need to pass an options argument so we only download a snippet.  If you don't do that, you are doing something wrong.  downloadBodyReps is the one that is for full body part/message downloading. XXX rename this family of methods.