You don't need them... it's all about driving technique and what you are used to. I have both nitro and electric 1/8 and both work fine. Mechanical brakes are an advantage in some situations, but it's not a big deal. On very tight tracks (lots of 180 turns etc) it's nice to have a rear bias brake setup, but most 1/8 tracks are big and wide. Brakes are overrated you should be rolling corners and barely using brakes.
Actually, as I stated before, I DO need them. I race on a Tight, Technical track, with several tight turns. So the kit for big open 8th scale tracks, won't work FOR ME. I'm not alone, that's why several people are building them with mechanical brakes. I understand that it works for some people, and that's cool. It won't work FOR ME. Besides, I race a Losi 8ight... I know how to roll through the corners...that's how I keep placing in the top 5 at my local track.
You can't roll fast enough through a tight 180, you have to "Turn and Burn" to hang with the guys at my tack.
An 8th scale buggy without adjustable brake bias is ill equipped for racing. I want the brushless system to act almost exactly like a nitro motor. Free wheeling when off the throttle. The brakes must be mechanical, and biased to the rear.