With the loss of a generator on a CAT 3 approach on the 75/6 you will lose your fail operational capability (unless it happens below AH) and may have to go around unless it happens early enough in the approach to rebrief for a DA versus an AH.
(Decision altitude versus alert height)
Having the APU up and running will normally provide for a no break seamless transition of electrical power and will allow you to continue using the same minima.(You still have 2 generators)
A big plus on low vis approaches and it is standard procedure at my airline to start the APU on the approach checklist. 10000 feet seems premature.
This is only on low vis approaches though, otherwise we just start it up after landing.