From my sons, who both worked in the kitchen at Ruby Tuesdays:

The carhop server receives the same pay as a server with tables (less than $3 an hour). He/She therefore is expected to earn tip income. Duties include taking the order, making sure that the order is correctly filled and delivering the order. Most patrons tip, but often not 15-20%

My opinion:

The service has value, but not equivalent to serving a table. I generally tip 10% and may adjust that amount in future visits if prior service wasn't up to snuff (assuming the same server).