Tanks having a heal makes sense. PLD has a special heal because they can cure others (and still heal themselves for 50%) but that's just to make it unique. Healing is preservation, so it fits thematically with tanks who are guaranteed to take damage. For everything except Extreme/Savage encounters, DPS should never take damage unless they make a mistake, or raid-wide damage (should never be enough to kill you outright though), therefore DPS having heals is to cover for a mistaken mechanic (hence long cool down, limited cure potency on Second Wind/Blood Bath/Life Surge)