@BelfastBre @JMG745 @subjugatedsuzi @AmandaFBelfast @ElaineCrory By-elections aren't compatible with an STV system. Take PBP in West Belfast: if Gerry Carroll had to resign for any reason, a by-election would almost automatically gift the seat to SF.
We've a party system with STV: co-option preserves party balance the electorate voted for.