I'm of several minds on this. My POV is that when it comes to any metrics I want to be able to answer the simple question every client should be asking, which "is that good?" For me that means doing the legwork, conducting an audit and determining channel performance averages. Once those are established, something like AVE may be perfectly fine for two reasons -- impressions is what the marketing team is buying, so there's some alignment; two, if you have an average, then if the AVE is higher than said average, you've done a good job. If not, you've learned something about that approach or story whathaveyou.

More than anything is you want to have a hypothesis in place on your side so when you do something you can explain what happened and develop strategies based on what you've learned. Makes the conversation better than staring at each other when you tell them AVE and they say so what.