The wiring kit gives you several different lengths of wires in kind of "standard" colors....The instructions tell you to wire the red one to the power, white to the ground, etc. It also includes the right connectors and stuff.
I happen to think you're gonna spend more than $5.99 or whatever it is on supplies, not to mention your time, gas, etc. You're gonna have to buy some kind of multi-pack of connectors, 10ft roll of wire, etc. I also haven't had any problems with the relay provided in the kit.
If you've got connectors and stuff already in your toolbox, skip the kit. Maybe buy some extra wire and consider the relay based on other's opinions. But being that my toolbox had NONE of that electrical stuff at the time, I found the wiring stuff useful.