http://weeklywonk.newamerica.net/articles/want-paper-plastic/ 

To summarize;  

Plastic bags don't kill fish or fowl.  Plastic is cleaner.  Plastic doesn't adversely affect the poor.  Plastic is reused paper isn't. Made primarily from natural gas so not a resource problem.  Only .4 percent of municipal solid waste is from plastic bags.

Lastly not mentioned is bagging your milk in a plastic bag means getting it to the house without the bag ripping apart.