CPM can drive your costs down on a highly targeted niche. Let's say you want to target a female tech journalist living in SF and using Slack, you'd rather use CPM than CPC as you're pretty sure there are only 20 people that match the criteria and even though you'd pay 50 USD per 1000 impressions, you'd only pay 5 dollars to ensure each person in this particular niche sees your ad 5 times!
I can count on one hand the number of people in this industry that like hard work.
It's not just the wrong metric, it's a fundamental misunderstanding of technology: for example the "viewable impression" as being anything other than software that is telling other software that it's on the screen...
It's like paying programmers per line of code written and then getting bloated and shitty code.
You need to be doing CPA (Cost per Acquisition) deals not CPM or CPC.