For the most part, the threshold for use of a relay in motor vehicle applications is determined by the circuit designer/installer. While its true that I have never designed a relay or any other switching device, I have installed thousands.
As an automotive technician, I am pretty much limited by the devices that are commercially available and easily attainable.
Over the years there hasn't been much I haven't encountered as far as wiring is concerned. From simple "add-ons" like we're talking about here to complete headlight to taillight jobs that included an MPFI engine. Some customers want micro switches located in the ashtray to control NOS systems, including bottle heaters, so they are easily concealed. The NOS activation is then triggered by the horn button. (In conjunction with the WOT switch and RPM trigger, of course. )
Wether the circuit is simple or complex, there is more going on than meets the eye. If memory serves, a 55w halogen bulb creates about a 250w load at the switch at initial contact. While that may only be for a nanosecond, it still happens. In MY opinion, there is no substitute for high quality components and high quality craftsmanship in any endeavor.
I may only be a backhoe operator, but I am a GOOD backhoe operator.