Using them and so many other starships in battle was an appalling waste of resources, since they were designed for exploration, research, and only given weapons for defense.
And with so many design flaws. I still feel numerous conflicting emotions* when Sisko tells his officers how oh-so-great it is to get this ship, a prototype, specifically made to fight the Borg, but it was discontinued because they didn't add enough bolts so it was close to flying itself apart. So they give it to that forgettable little corner of deep space...

* no worries, I hate them all...

Say what you want about the Defiant, every aspect of her from warp core to outer hull was designed and built as an implement of destruction.
Trek has been somewhat uneven** regarding the Fleet's purpose, but the Borg did seem unstoppable and how would one otherwise stop them?
** read: grossly inconsistent