Spend energy to get combo points -> finishers are fun!
Spend runes to get runic power -> deathcoil/froststrike, which can also give you more runes. Fun!
Spend focus -> get an empty focus bar, which means you have to stop moving and use casted shots that aren't fun.
Out of the 3 resource systems listed above, focus feels like the only one that actually punishes you for using it.
Maybe the first 1-2 steady/cobra shots after you're below 20 focus could be instant cast? Or the first 1-2 steady/cobra shots after you're below 20 focus could get a damage bonus.