Power Infusion has been +haste (not Troll Berserking's casting speed, actual +haste) since before Shadow's DoTs scaled with haste, mid Wrath.
Shadowfiend does more damage in one burst than Mindbender, is on a 3 minute cooldown, and requires zero talents. For a single GCD, in this purpose, it can be construed as a burst cooldown. A kitable burst cooldown, but a burst cooldown nonetheless. If you choose to spec Mindbender, you get one that does a little less damage, but a lot more often. But not spec'ing Mindbender does not remove the fact Shadow has a cooldown, and Power Infusion would give a second one if that's what you're after.
We do need a buff to sustained DPS, yes. Very much so. The point is, this is a bigger priority (Mind Blast not scaling with haste, no resource generating increasing with gear, or with bloodlust, or with anything, overall lower scaling with stats).
Just flicking the switch on Mind Blast's haste cooldown reduction (was in for, what, one build on beta?) would be a very decent start, and would actually do more for Shadow than giving Archangel's +25% Mind Flay damage back (which current PTR build Insanity does +100% mind flay damage, go figure).
Say it with me now guys, WE DO NOT WANT TO BE RET PALADINS. Say it again, come on. WE DO NOT WANT TO BE RET PALADINS. They have a lot of cooldowns, and because of it, when their cooldowns aren't up, their damage suffers. It's not fun to only be able to feel awesome for 18-25 seconds every few minutes, and suck the rest of the time. We need our baseline damage issues fixed first.