If the auto companies get a bailout, then they can continue to sell cars (GM's revenue is on the order of $150B) and that revenue can be used to continue to pay the workers, the suppliers, etc.
If they go under, then all the money to support laid-off auto workers has to come from the government.
The wage of an auto worker costs from $1200/week to $2920/week (depending on whether you believe the $30/hour number or the $73/hour number floating around).
http://www.michigan.gov/documents/uia/EUC_Fact_Sheet_120_240...
Additionally, auto workers have an incentive to get off unemployment; it pays less than market rate. They have no incentive to stop working for GM, which pays above market rate.