I got to read Gary Angel's post on Measuring Engagement without Excuses which is based on Avinash Kaushik's post on the same thing.  I also took a stab at Avinash's post two days ago in Engagement - Avinash Kaushik "It doesn't exist".

Gary says something at the end I want to quote here:

"…Add it all up, and I think the biggest idea I’d take away is that if you are going to call something Engagement it probably shouldn’t be reducible to a single simple metric (good point – I’ve made that mistake) and you should probably make sure you are comfortable with the trade-off between the long-term economy of collapsing multiple metrics with the price you pay both in explanation and loss of clarity (this isn't in the original post at all but I think it's what ultimately matters).

As is so often the case, the real-world complexity of being an analyst is rarely reducible to simple rules of “do this” or “do that.” Reporting is much more complicated than most people ever realize, and it involves an unending series of trade-offs to which there is no perfect answer. Can an Engagement metric be misused as an excuse? Of course. Can it be immensely valuable? Of course."

I think what I'm responding to, and what Gary responded to - its oversimplifying engagement  to the point where it's no longer worth measuring. 

Life can be simple, but no one really believes life is easy.   In Engagement is no less meaningful just because some think it can't be measured - nor will less people be asking for that series of metrics anytime soon.