My first thought about seeing a card that costs Exert was..."Great,
doesn't cost me a thing!" But I was wrong. Exert costs much more
than that.
It ends your turn. Most games last 7, 8, maybe 9 turns. Longer
than that is unusual. This means you will end your turn about 8 times.
Consequently, you will get to play exactly that many Exerts.
Most abilities based on your champion, such as Basic Attack, will also
cost Exert. If you are playing Idiris and plan to Fireball a
lot, then don't take too many Exert cards. You won't use them.
In other words, every turn you are choosing between using your
Champion's Exert or a card that costs Exert. Having a card that
costs exert in your hand while always choosing your Champion's ability
means having a dead card in your hand. It's a tough choice.
The number of Exerts in your hand should be based on your
strategy. And your strategy should take into account how many
turns you want to play - a long game or a short game. If
you are running some high powered summons, like an 8 Chi Lightless
Tibicena, then you will probably want a lot in your deck. But, you
could get away with fewer Tibicenas if you use Underworld Knowledge
(UK). But UK costs 1 Health and Exert. Turns that you want to
use UK hoping to pull a Lightless is a turn you won't be throwing a
fireball. To determine the optimal number of Lightless and UKs,
you would either need to practice with the deck or calculate some
hypergeometric distributions.
Either way, don't take too many cards that cost Exert.
