Why X cards?

Slay the Spire has cards (which for this post I am calling "X cards") that cost all of your remaining energy. The effects are generally proportional to the energy spent. For example, Silent can get the card Skewer which costs X energy and "deals 7 damage X times." If you have 3 energy and you use the card it deals 7x3 damage, if you have 2 energy it does 7x2, et cetera.

I haven't been taking these cards. X energy could be a lot of energy. On the other hand X energy also means if you draw this card then you won't have to waste any energy that turn for lack of a cheap enough card to play. And 7 * X damage could be pretty good.

Is this enough? Enough to be worth a try, I guess.