The current I'd expect is fairly decent being that it's a 6V system, right? Even if it's a fairly low power light, say 35W, that's around 6A. Well, that assumes it's an incandescent bulb, if it's LED or something then it would be quite a bit lower.
I like to always use relays, although it's not like the runs are long on a motorcycle. But it does simplify the high current side to just two wires and makes the control side wiring lighter, which I would assume on handlebars might be important since trying to connect 14AWG would be a PITA compared to 22AWG.