“Disregarding the report sight unseen because Microsoft sponsored it” isn’t really applicable here, it’s a distraction from the point: A report that favors Company A’s product over Company B’s product is not as reliable if Company A paid for the study.
If Company A pays for a study that favors Company B and then publishes that study, I think everyone would believe it. So your sentence isn’t accurate, it is the outcome combined with the payment that causes doubt.
When IBM publishes pays for studies that favor them, I assume that IBM made the rules, tried to make a comparison that they would win, and then paid a large sum of money for it. If the company that did the study ended up favoring IBM’s competitor, IBM could tweak the rules a bit and do it again, but how many times would they do that before deciding to use a different “objective third party” to do the next study?
I have never fully believed a study that favors the company that sponsored it because I have never seen a study that ruled in favor of the competitor.