A shield will give you at best, 10~15% damage mitigation, and that is largely irrelevant since most of the low level content is a joke, there is no reason to reduce the damage taken if your healer can handle it. If you outgear/outlevel the content, there is no reason for using a shield at all, except for a few cases, l can recall from the top of my head King Gordok hitting harder than raid bosses, in this case you can't survive without a proper tank gear.
I'm not particularly saying that a resto druid tanking is better than a prot warrior, but with the proper gear they can do the job just fine. Most people still have the retail mindset where you cannot perform a certain role if you are not specd into it, gear is far more relevant than talents for most classes, specially for spell casters.