Implied, but not stated so far, is that an engine failure can contaminate a pack and the pneumatic ducts, with oil and other nasties, which could lead to fumes in the aircraft.
Therefore, one should think very carefully before switching on the same-side pack of an engine failure, or even of opening the cross bleed valve to feed both packs from the good engine, because contamination could then get through. Be warned: the ECAM does not consider contamination, and may invite you to configure the pneumatics to feed both packs.
Last edited by Uplinker; 5th Oct 2013 at 11:28.