In reply to dregsy:
Monos on rocky mixed all the way for me, for the reasons described.
On ice it's less clear-cut and will depend on the solidity of the ice and the quality of the secondary points. Some crampons have more agressive secondaries than others and this extra contact feels more reassuring to me. On cruddy or soft snow-ice I'd go for duals every time, ideally old-style flat front-points.