This mostly applies to pvp where it’s a death sentence to cast most spells outside of precognition, but there are cases in pve as well where some instant spells will hit significantly harder than those that have a 1-2 second cast time.
Fire mage is a great example, where fireball, a spell with no proc to cast instantly, does less damage than spells that instant cast. A great way to help, in my opinion, would be to make a spell like pyroblast deal triple damage if hard cast due to its long cast time.
Incinerate for destro lock shouldn’t be doing pitiful damage, either, but it does far less than a shadowburn or even a conflag.
Living flame for evoker is probably the single best example of a spell that is completely useless as a cast and is pretty much only used with an instant proc. It sucks because casting should be rewarded, not punished.