The Reapers were mindless, the Zerg were(mostly) mindless, yet does that make them not interesting?
When it comes to mindless entities, their thought processes do not need to be interesting. The interest that comes from these types of entities is not in why they do what they do, but the danger and threat they pose to the galaxy at large. I did not care that the Zerg or Reapers had a purpose beyond their actions, only that their actions could lead to the destruction of the galaxy. It creates a interesting threat that can help create conflict not just in battling the entity, but in the conflict that can rise as refugees and such as they flee the entity.
Just because a entity is mindless, does not mean it is not interesting.