I agree. I also use Bibtex string variables, mainly for the same reason: the conference "International Conference on Software Engineering" I store as "ICSE". This makes it much quicker to sort, because every conference is seemingly an "International Conference". This however screws up Mendeley's statistics, and I have to write a parser to convert the Mendeley output to the proper string format (i.e., remove quotations). It would be nice to have a "venue nickname" category. An API would be even better.

A workaround: say you add a document to a collection, and want to update your Bibtex file for that collection. To trigger the re-generation of Bibtex, go to that collection and toggle the 'unread' icon (the bullet) (on/off works fine). That should be enough to force the export.