The only time I could see setup being useful is if the normal number of hits you get isn't enough to damage an enemy, even though you can get a lot of them. In other words, an enemy with great damage soaking ability but poor blocking ability - some troll tank builds come to mind, not much else.
If the enemy doesn't have that great damage soaking ability, you will be better off doing normal attacks instead of wasting a successful attack on setup. If the enemy doesn't have poor blocking ability, you won't get enough net hits on your first strike (the setup one) to make enough difference for your second strike. So to me, it is a maneuver, like Full Offense, that has a very niche role.
Of course, if the group commonly runs into heavily armored but low skilled mooks, it might be far less of a niche role.