could you not get rid of the need to preference every single candidate? doesn't seem to do northern Ireland or the republic any harm to have a few candidate elected on not quite a full quota and some people's votes becoming exhausted
Parties gain when they can control the destination of their voters' preferences.
Consider that intra-party transfer rates in those other STV jurisdictions are not very large: typically 20-45% of voters will stop preferencing, or preference a different party's candidate, when a same-party candidate remains. One author cites 67% and 58% intra-party transfer rates for Fine Gael and Fianna Fáil respectively in 2011 (
http://www.ucd.ie/ibis/filestore/wp2011/109_hutcheson.pdf).
