I think this is a good idea and I see now downside to it.
Incidentally, I found that some things don't add up until the last second, such as Hope, which needs to count your light-emitting creatures. Also, cards like momentum seem to catch me off-guard as well in terms of damage, even when I do have a shield. Another example how antimatter will add to your health instead of subtract (I think it just zeroes it at the moment). I think we have to revamp the way the health bar calculates before anything else first.
However, I do agree wholeheartedly with this idea, even if I'm unlikely to use it.
I, for one, am not very trusting with some matters, which include the possibility of miscalculating and losing.