Well partly because of the TP for MP cost of impact, secondly because casting impact also gives a fat -20% stat down which, at its shortest duration which will be always on Aminon and other high level targets, will last ~19sec.
Even if you have both geo and rdm casting impact, there will be times where it won't be up. Having it up for more time means more damage for the party as a whole.
Also when I was doing Aminon I was the only one casting impact, so minimising recast was a very big dmg boost even without the TP gain.
As always, things vary depending on play style, the rest of the group and so forth like Buukki said.