No, they're long ranged, and can kill the Rebels as they flank them, which results in them being spread farther apart and losing strength in numbers. They're meant to draw the enemies out of their bunker during the seige stage of an assault.
They followed the Tarkin Doctrine: Rule not through force, but the fear of force. The Death Star was to be the ultimate form of the Tarkin Doctrine. "Oh, you feel that what we say is tyranny? Just a reminder that we can destroy your planet in about 5 seconds."
one thing about those i found funny was that if they had just stopped trying to move when they got cabled they could have just been a firing platform and just rained death from afar while they sent someone to cut the cable.
They didn't have super defenses around the one weak point on their super death moon of destruction and despair. Do you think they'd train pilots and operators to have common sense or do you think they'd indoctrinate them to the point that the mission is the only thing on their mind and survival is that little speck in the distance?
These were more a deterrent than a actual war machine, Once the rebelion started a real battle they reverted to better designs from the clone wars and eventually fixed the walker with the gorilla class walker that we see in episode 8.
And then these bunker busters roll in.
The Empire ruled with two things, fear and firepower.