except that it will not only be used in the "well, im going to lose anyway" situation. that is a good use for it, but it is not the only. could add a self damage component to it, possibly, to lessen the "well, i only have 10hp, im going to lose anyway" use.
also, just a note, the win/loss extension is like saying anything bad is comparable to being hitler (well, that teacher is a little strict, so he is not making my son happy, which takes him from the good side, so he is on hitler's side of the isle). extension to extremes aren't always the best way to look at things, even in the sense that you portrayed. not to mention the win/loss card would be dumb, plain and simple. this is not guaranteeing a win nor a loss, it may help in either direction.
an alternative, significantly less powerful, less interesting, version would be doing 10-20 damage to the loser of the coin flip.
-or have it be under a sundial like effect where damage to the other player (depending on who wins) cannot be done during the extra turn, or something.