It's not a good idea to judge information by the source. That's a reverse appeal to authority.
What better evidence of honesty do we have than by the apparent lack of bias of the source?
I grant you it's not perfect, and sure the involved parties may have access to real facts about the situation of the time, but the rest of us only have what the MSM (and a little non-MSM like Alex Jones, TYT, and RT that make us feel dirty) have reported on plus a tiny bit of our own mental product that we can piece together with what we've learned in these places.
So is there a point in talking about anything, ever, when you don't have all the facts?
It doesn't matter who is saying something, what matters is whether it is true or not.
At least if your source seems to have nothing to gain by the "facts" it is reporting, your chances of it being true are better than if otherwise.
Do you have a better way to discuss the situation without having all the facts?
Btw, I hate when the greenies do this to anyone who has an issue with AGW. They immediately attack anyone disagreeing with AGW as being in the pocket of big oil.
That's because most greenies are uninformed children speaking out of fear. When I argue for the case of AGW, I always post in-depth research results as evidence. (Which ironically can be said to be invalid because AGW proponents always make the case that those professors are biased to come up with that evidence!)
What really matters is the quality of the criticism, regardless of whether big oil is behind it or not.
Establishing a motive can be helpful though. I agree it is not to be used as the sole reason to make an argument... But if you know the motive behind a standpoint like "AGW = Fraud" then you can make a hypothesis on what else they had the ability to do and can do in the future, and then test all of this over time.