Philippe, HCA is really more beneficial when used with Fibre based papers. This is because some of the equilibrium products formed between the silver halides and the thiosulphate are only slightly soluble and can form weak links with the cellulose in the paper base. With fresh fixer this isb far less of a problem, but with partially used or exhausted fixer these silver-sulphur complexes remain in the paper, particularly in the base. This is the reason that many people use dual bath fixing for fibre based papers. It's these intermediary complexes that cause staining etc on poorly fixed prints. The chemistry is quite involved as there are many possible intermediary complexes that can form.
A wash aid, (HCA or 2% Sodium Sulphite or Carbonate, or a mix), helps to tip the equilibrium balance towards soluble complexes which significantly improves the washing process.
It's easier to wash the complexes from film, & RC papers where there's no contact with the actual paper (cellulose) in the base. An HCA bath will cut washing but not by a very significant amount, and wash times for films & RC papers are relatively short anyway.
Ian