I also am aware of a situation where a pair of business consultants who were meant to be assisting with a software project were diverted (at full rate 1200/day) to assisting with redecorating an office.
I was directly involved, oppositionally, to a pair of business analyst consultants who tried to get a customer of mine to change their (admittedly terrible) vendor selection by repeating security concerns over and over again in the meeting. They never actually got to the point of analysing said terrible vendors terrible integration practices or costing up a migration path. They just banged on about security and contacted us separately after the meeting asking for more details about the security situation.
Basically you get out of it, what you want to get out of it. It depends on the consultant, their education, and the terms of their engagement. I don't know if statistics would be useful in this scenario or how you would control for wildly different outcomes.
Like someone else pointed out, if people are hiring them in order to provide cover for decision making, then maybe the whole thing being a charade is the point.
Well, McKinsey still existing? Too much influence. Otherwise they would have gone like so many other consulting companies.
You are now a fully trained management consultant. (Alan Johnson, Peep Show)