Well for starters, like any sort of 'rush as many of these cards as possible' deals, sometimes figuring out what else to buy is going to be very valuable. Sometimes it's obvious, but sometimes there might be other things to supplement the rush which require a bit of creativity.
And secondly, even in a boring IGG/duchy rush mirror, figuring out when to transition from IGG's to duchies is very important. Often, you want to start grabbing duchies before the IGG pile is empty. Maybe when there's only one or two left. The reasoning being, well, at that stage of the game, handing out an extra curse and winning the IGG split is probably not as good as winning the duchy split 5-3.