Arimidex will suppress the estrogen-related water retention and bloat, which many people enjoy and consider to be a valuable part of their gains. With 1mg arimidex ed, on that cycle you would stay very lean and gain only the quality muscle mass part of the gains, eliminating the bloat.
For myself I only want lean gains, so I always use arimidex. If you just want to get as huge as possible, you might be disappointed. If you only want quality muscle gain and want to stay dry and ripped, you will like the arimidex.