Gents, all quite simple... Must agree for most part wirh schnowzer.
No problem flying below GD, as long as you stay out of trouble, which is avoiding the VLS.
When would you do it? For example on an extended downwind where control asks you to reduce to below your green dot. Instead of getting the drag already out (an make it uncomfy for the pax with only slats out.. lol), you could just reduce to green dot. The moment you get your base turn, you get your flaps out and start turning... In my opinion that's great airmanship.
About the comment that nothing is written in the book and the fuel pumps etc etc...
When you switch off a fuel pump you'll get a light that says OFF, while airbus has a dark cockpit concept.. on the other hand, nothing prevents you from vlying below GD, even the AP will do it in selected mode. It gets dangerous near VLS, so that's where they built the protection, hence the name VLS.
MR8