I wouldn't call PI the worst talent... The other two talents boost throughput, PI boosts throughput and mana longevity (which is obviously not an issue right now but with a new raid comes new struggles). Divine Insight looks very nice, but I don't think I will be taking it for most encounters. Maybe if there was another disc priest in the run but even then, I run 25man.
PI will still be useful for 25 man spirit shelling... and yeah i can't really say i'd use much of DI in a 25. maybe in a 10 man with heavy PWS spam where you end up weakened souling most of the raid? also, will DI'd PWS add to an existing PWS or will it just overwrite? if they added together, that could actually be a useful tank saver combo of defensive penance into a double PWS.
also, it's probably nice for very offensive pvp play, since you can offensively penance and then you're guaranteed a PWS on whoever needs it. the tiny size of pvp teams makes it seem like that's where this talent would really shine.
it's a significant boost in absorb. i could definitely see using it in a fight like megaera, where you want to have spirit shell and PWS up on people going into rampage, but you might get your absorbs burned through before weakened soul falls off.
i also see it being much more useful in 10s than 25s, especially if you run with 2 discs in a 10.
The change to haste instead of cast time reduction sounds very interesting (for Power Infusion). I'm assuming this means that it now boosts our Shadowfiend/Mindbender like other haste increasing effects do, making Divine Insight much less attractive for most encounters, in my opinion.