That's flawed logic, because you can't choose where it's "focused". You might waste your rare crit on a 100 HP enemy that was about to die anyway, or on a full HP ship that gets healed back up right after. In the end, crit perks are for random damage, while damage perks are for steady damage.
It's not necessarily true that damage perks are always better "on average". The % increase of damage perks depends on how much % damage increases you already have for Blast Cannon, because different sources stack additively. For example, if you have no boosts at all, then an 8% damage perk gives you 8% damage, but if you already have 50%, then an 8% damage perk will only give you 100*(1.58/1.5 - 1) = 5.3% damage increase. So, the more advanced your crew training is, the more attractive crit perks become.
I did a quick calculation for lvl 30 Bhurt. This showed that crit and damage are roughly equal at that point, against a non burning target: 18.6% avg dmg increase due to damage perks vs 18% due to crit. If you also have 5% point damage from Brock and 6% from Buzzkill, the damage perks drop to 17.7%. Meaning, with maxed out crew training, crit perks win, though the difference is tiny.
Also, against burning targets, crit perks get ahead even more. If you're running burning yourself you should consider +dmg vs burning target perks of course, and then this whole comparison is irrelevant. But if you're not carrying a burning source, and end up in a match where your teammates are burning enemies, you get an additional 20% dmg, so with a maxed out crew training your 4 epic cannon damage perks would only provide you 15.9% dmg increase, assuming that damage against burning targets stacks additively like everything else.
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