Quote from "How to Brew" ...when using fully modified malts with a large proportion (>25%) of unmalted grain, e.g. flaked barley, wheat, rye, or oatmeal. Using this rest in a mash consisting mainly of fully modified malts would break up the proteins responsible for body and head retention and result in a thin, watery beer.
I cut out the part discussing the usual moderately modified malts as we know that already. If you have other things in the mash that can benefit fro the rest that's good, but if that is not the case, and you are using standard modern malts, it just doesn't make sense, and as above, may work to make your beer thin and watery. Depends on what you are shooting for :-)