This is something that you really only want to play while you start in hero form. It costs 3 resources, but you should almost always be able to pay that unless you spent cards during the villain phase or an encounter card forced you to discard something from your hand.
So essentially what you do is replacing a hand of 4 cards with a hand of 6 cards, then you flip back to Hero form and you deal 2 damage to an enemy.
In other words for 4 ER you gain 6 ER (with a net gain of 2) and deal 2 damage. Moreover this also gives you a chance to recover without ending your turn in AE form, and/or a chance to use Legal Practice, Superhuman Law Division or any card that can only be used in AE form.
There aren't really many reasons not to play this card every time you can. One reason may be that you really need to end your turn in AE form, but those should be pretty rare cases. Another reason is if in your 4-cards hand there's one that you really want to play. However this would assume the worst scenario where you don't have anything that can boost your resources. Helicarrier, Genius/Strength/Energy, Focused Rage, Quincarrier, there's a lot of options that can allow you keep one or two cards in your hand and play split personality anyway. The effect doesn't really changes, you'll still have 2 more cards than what you started with.