If you're running TiNi shock shafts the the RPM ones arent as big of a deal otherwise they do offer some protection and personally I think they look really good. I use em on some of my traxxas rigs. I dont generally use shock socks though.
The RPM Shaft Guards are hard RPM plastic that helps protect your shock shafts from taking direct impacts, The soft Lycra covers help keep the shocks from getting dirty. Use one, the other, both, or none. Personal preference.