While I do agree with you that the opportunity cost based analysis is better, I do want to point out that for this context (or any context where this might be the case) the opportunity cost of doing an opportunity cost based analysis might be high enough to warrant opting for the inferior but easier/cheaper cost based analysis.
Of course there might be an easier way to quantify the opportunity cost that I can't think of.
"Is the the best possible use of our time, collectively, and/or of each of us, individually, for the organization and our collective goals?"
I suspect you'll find that more effective people tend to recognize when they're in a meeting they 1) don't need to be in and 2) which is keeping them from doing something more important.
Socializing, strengthening group cohesion, and other organizational (as opposed to individualistic) objectives may mean that even if you personally perceive greater benefit at being elsewhere, the organizational / collective goal may still trump, which is why that's included in the metric.
The concern isn't to do a thorough evaluation where that's not itself cost-effective, but to use the correct basis for measurement. Deciding on imperfect information is perfectly acceptable. Deciding on the wrong basis should really be avoided where possible.
Of course there might be an easier way to quantify the opportunity cost that I can't think of.