Their offensive potential is mediocre--no more dangerous than a comparable points value of Ork Shoota Boyz, honestly. I mean, it's nothing to sneeze at, but low BS lasguns just aren't the end of the world.
Their primary problem is the fact that one Commissar negates their huge morale problem and they become this super-cheap immovable block. Now, they still die like Eldar Guardians, but when we combine their iron morale with the fact that there are very, very few effective horde-killing weapons anymore, and we've got trouble.
The reason units like this are in is simply because Blast and Template weapons--the age-old horde equalizers--are now pretty damned terrible at their job. Time was a couple pie plates would put such a hole in a unit of conscripts that you basically broke its back. A big unit like that couldn't effectively space out and still be a viable unit, so they always were a bit tightly packed, and the old scatter mechanics made it almost impossible to miss (which makes sense).
Now we've got stupid Basilisks or Demolishers killing, what, 3 guys? No wonder people love hordes. They have no paper to their rock anymore.