On my paladin in the mid-40's in my healing set I had about 50% cloth armor, 30% leather armor, and 20% mail armor (no plate) and I was using a held in off-hand item. I healed fine, as long as I wasn't getting hit. Only if you're taking hits (i.e. soloing, PvPing, tanking) do you really need a shield. Or of course if the shield has better stats.