This reminds me of the "debate" that noone has PROVEN that car wax helps prolong the life of a car's paint finish. A better way of putting it is, although the wax/sealer helps, is it necessary? The answer is no. However, yes, you can seal concrete with a waterproofer that is available at any Home Improvement store. Although they are not strictly necessary, they obviously do help. They provide a protective layer over the concrete that helps it withstand weathering. Most public sidewalks are not sealed, of course, which partly accounts for why they end up cracking so easily. A properly prepared, poured and sealed slab should not crack or peel under normal circumstances. Also, some concrete mixtures are weaker than others, and thus will require sealant in order to last, whereas others are fine all by themselves. The default answer is that is cannot hurt and will probably help quite a bit.
[This message has been edited by Lawrence (edited December 12, 2000).]