Multi Energy is the toned down version of Rainbow Energy: meet all Energy Type requirements without hurting yourself. The catch is, of course, that if there is any other Special Energy attached, it only provides (C). With proper use, this shouldn't be an issue. This card wasn't designed to replace Rainbow Energy in all decks, just a few: namely if all you need is to meet one off color requirement, you run these. The only time in such a deck it would be bad is if you only drew into copies of Multi Energy and thus were unable to attack because of the draw back. Simply put, that's just bad luck and is a part of the game, and not a reason to ignore this card.
If you're using Pokemon delta, then you just use delta Rainbow Energy. If you're using a non-Pokemon-ex Evolution and need more than one Energy of all types, you'd use Double Rainbow Energy. For just a single Energy need of an off type, you run Multi Energy.
Ratings
Unlimited: 3/5 - It isn't actually a bad card, just rarely needed.
Modified: 3.5/5 - Still isn't needed that much.
Limited: 4.5/5 - Often needed, and when it is it makes running decks so much easier.