Who should get PI?
It depends on the fight.
And a bunch of other stuff.
This tool graphs the difference in average DPS among the top 100 parses for each spec with PI and the top 100 parses without PI. In other words, this graph shows the average and maximum gain in DPS from getting PI over 200 parses.
FAQ
Q: Why does it say my spec doesn't have a significant difference with PI? I do more damage with PI.
A: Everyone does more damage with PI. This chart shows the delta in the top parses of that spec, which has interfering variables such as group skill level. In more useful terms, it means low skilled groups PI poor targets more frequently, on average.
Q: Why does it say my spec doesn't have a significant difference with PI? I do more damage with PI.
A: Everyone does more damage with PI. This chart shows the delta of the top parses. In other words, the best outlaw rogues rarely get PI because the people they play with know that there are better targets.
Q: How frequently does this update?
A: Once per day.
Q: Why is this significantly different from Bloodmallet?
A: Because 5 minute patchwerk sims don't accurately represent a boss fight. This site doesn't sim anything, just looks at logs. Bloodmallet additionally uses the default character profile for each spec, which is not representative of what is good in raid. For example, the Bloodmallet fire mage profile runs frostfire while there isn't a single frostfire mage in the top 300 parses for any boss this tier.
Q: Why don't I see a Rank 1 DPS Difference for <spec> on <fight>?
This indicates that the rank 1 parse for the fight did not get PI, thus the delta is negative.