From what I understand, slurred speech is caused by weakening and atrophy of the muscles that are involved in movement of the mouth, tongue, and vocal cords. These are voluntary movements, so the motor neurons that operate these muscles degenerate in the course of ALS. Excess saliva may cause some difficulties as well, but I think that would be secondary to the loss of motor control which we use to speak.
I met a guy at a support group meeting that had bulbar als,,the only thing affected with him was his speech,,he had more than saliva,,it was like a water faucet he couldnt turn off,,he was wiping drool every few seconds. Not sure he it affected him eating or not.
It was my first meeting i ever went to and i didnt ask questions,,,this is just what his wife said.