Makes no business sense not to qualify all pilots to cat 3. Rosters are released 6 weeks or so in advance. You don't know where will and where won't be in less than cat 1 conditions that far in advance (24 hours if you're really lucky) so putting together a flight crew with that short notice impossible. Only need 6 approaches in the initial conversion sim (plus 3 practice auto lands in the real a/c) and minimal recurrent training.
There are certain items that only certain pilots are qualified for in an airline such as cat C airfields and etops but not adverse weather.