You shouldn't NEED one, but they're nice to have. If they do what they're supposed to, you should be able to gear up and get some more speed while keeping your electronics cooler. I had some on my Slash 4x4 before I sold it, but I don't use any on either of my 2wd Slashes currently.
-Slash VXL 2wd Strc LCG, TRX Big-Bores, homemade chassis cover & some RPM upgrades
-Slash VXL 2wd, Pro-Line PowerStrokes, homemade chassis cover & some RPM upgrades.
Tires, bodies, batteries and tools shared between both.