The only real pro for assembly line justice is the fact that it does not cost as much or take up as much time as "true" justice might. This means that society can have some measure of justice without having to pay too high a price.
The cons are more apparent. Assembly line justice, one can argue, denies true justice to many defendants. It treats defendants as if they are guilty and disposes of them as quickly as possible rather than giving them the same kind of chance to defend themselves that people get when they are able to afford their own attorneys who will ensure that they get a full trial.
So, assembly line justice represents a trade off in which society gives up protections for the accused in order to get more efficiency and in order to pay less for its system of justice.