Unless your rear derailleur gets bumped or otherwise pushed inwards, such as during transport or while parked. Having it might save you from having to replace some spokes. People derisively call them "dork discs"; I use them because they could save me from a hassle and because I don't care what other folks think. I also use a Drop Stop up front; nothing ruins a nice ride like digging a greasy chain out of a tight spot.