Respectfully, I disagree.. Cume (avg persons) is a big part of the cost per point formula. It's why digital metrics are so attractive (effectiveness is another story). If I own two stations that share some DNA, ideally I create a story and can claim more of the market buy. It's why you could couple an RnB and Hip Hop duo or AC/HotAC, etc.. Audience duplication plays right back to reach/frequency anyway.
CPP is based entirely on the instantaneous delivery of the average spot in the time period being bought. Cume does not enter into CPP or CPM at all.
"Cume" is not "average persons". It is "total persons... at some time in the week".
Cume is the number of different people who qualify for at least one quarter hour of listening in a week. They may have cumed the station because they spent 10 minutes in a convenience store where the radio was on... but they did not listen at any other time. "Cume" is an abbreviation for "cumulative" and means the sum of all listeners, all hours, all days, for a tiny minimum amount of time.
Audience duplication means that if you buy a pair of highly duplicated stations, you get high frequency for some listeners and not high enough on others. That is why buys often do not duplicate a specific format unless the buy is going very deep.
Groups that cover a specific audience segment will sell the AQH in each of the age groups... and example being iHeart's three "wall of women" stations in LA covering CHR, Hot AC and AC. There is overlap, but they can very nicely cover an 18-54 or 18-49 buy perfectly. Or cases where Univision has a regional, Adult CHR and adult hits station in the same market where they cover a spectrum.
In the above examples, we see why AMP in LA or one of the other regional stations or adult hits stations in Spanish don't get bought... the buy is not deep enough, and there is too much duplication.
Remember, much of the buying for ratings-based orders is now automated. Agencies can specify things like "no format duplication" and optimization of the young, middle and older parts of the target. Salespeople can't make the kind of argument you mention because the transaction is done by computer.
Remember, cume counts one slice of listening as short as 5 minutes in a whole week to qualify. Agencies and other transactional buyers don't want to buy that kind of light listener. They use actual "how many people are estimated to hear each spot" for decisions.
For example, I am a "cumer" of Cleveland Indians home games. Yep, I went to one game in 1962 when I got my driving license just to prove I could do it. And that shows the value of "cume" in the buying process.