When the SRS light stays on longer than usual and goes out there is a fault in the system (duh, right?) that is not detrimental to the function of the airbag(s) (ETR may not work). When the SRS light stays on the entire system is deactivated meaning the airbags won't (shouldn't) deploy.
After the '90s scare of inadvertent airbag deployment (notice they were all domestic veh) its good to know that MB has one of the most safest and sophisticated airbag systems in the world.
I wonder why they call them airbags? Its not like you could inflate one and use it to put your groceries in.