My daughter is 32 years old and has worked at Sonic as a roller skating server since high school (what can I say, she's REALLY committed!) These servers are not like servers in a traditional sit-down restaurant, they are paid at least minimum wage to start and of course, like my daughter, the longer you've been there, the more you are paid. The roller skating servers also make more than the ones who don't skate - skating is not required, but encouraged. Therefore, I do not feel a need to tip them like I do a server who makes less than minimum base pay. So I don't.
Having said that, my stance infuriates my daughter to no end - she DOES fully expect to be tipped.