I think you are almost always better off with an evac class over a second healer. If you are in a competant group the rare extra add that a second healer would have saved you (IF) is taken care of by the scouts evac. You will kill stuff faster, and have the added utility spells the scout brings (smuggle, bard songs, etc), and if you are drinking anything but store bought crap, you really shouldnt have power problems.
Second healers are nice if something bad happens, but from a purely mathamathical powergaming standpoint, more dps + evac >>> second healer.