Heimdall is the most expensive non-variable-cost card printed. Most characters will need to dump their entire hand just to play him in alter-ego form, to the exclusion of other cards that turn. Not every hero can afford this luxury, and sometimes when you draw him you simply will need to do something else (especially when under Shadow of the Past or expert mode pressure).
The response is absolutely ridiculous in solo play or duo with a stunned villain, choosing the villain's boost card and your next encounter card, but is much more lukewarm in larger games where it will usually only affect the order of boosts.
However, what we have in this card is probably the first ally that you absolutely do not want to use to defend. His 8 threat thwarted over 4 turns is simply unparalleled in a single card, essentially twice as potent as an empowered For Justice!. Moreover you can decide to use his last point of health to instead throw a Haymaker. The flexibility might not rival Nick Fury, but the longevity certainly does, especially when you consider that this investment is worth supporting with other cards such as the ever- amazing First Aid.
In summary, don't write this card off because it is hard to use effectively. It isn't as flashy as Energy Channel or Shield Toss, but in this game that forces constant decisions about opportunity cost, the payoff can certainly be found here.