think if you want to include COP you have to prevent it for relaying information from the future, otherwise a smart caster will still get game breaking information.
Then it isn't worth the spell slot it occupies. Hell even Divination, which is two levels lower, can look 1 week into the future.
After thinking about it a bit more, I think I agree that Mindblank should block answers to direct questions about Mindblanked individuals. Per the "fluff" it probably shouldn't, but per the RAW it should.
I think Divination's usefullness is a lot less than COP, I'm uncertain as to its TO potential because of the DM involvement it might entail. I think COP is still useful if limited to the present tense, it can answer questions like:
Is anyone plotting to kill me?
How many people are plotting to kill me?
Are they planning to do something in the next week?
Which plane are they on?
Are they in a city/Which city are they in?
Is the most powerful of my malefactors more powerful than me?
Is my most powerful malefactor a Caster/Assasin/Fighter/whatever?
Do they have a strong Fort/Ref/Will/SR?
What toppings should I get on my pizza?
So even limited to the here and now I think COP is definitely worth its spell slot. Now if its blocked by Mindblank, like Legend Lore or Greater Scrying and that makes it too weak that's a problem with Mindblank not contact other plane.
@the mercenaries
100 level 1 mercenaries, why are we even talking about this? I think you could build a 20th level monk and mop the floor with 100 level 1 mercs, and I hate Monks. The Monk wouldn't even get any experience for doing so for heaven's sake!