Umm, its really quite a useful ship. I'm pretty sure designate targets does not force your ships to attack the ability's target. Even if it did, the ability enables your ships to do 40% more damage on that target, so it could still be worth while.
Its other ability restores 2.5 shields per second to one of your ships and increases its rate of fire by 10%, thus increasing the DPS by 10%, which is always a good thing.
In short, I would build more Hoshikos for repair than these guys, but these guys are great supports for battles with lots of caps. If you've researched them, why not use them