I love listening to different local Top 40 stations in the US and elsewhere. However, I think it's too early to tell what the effect of nationalizing music radio will lead to.
When looking at radio's past, we see that prior to TV's rise, most of radio's ratings-grabbers were national radio shows that aired throughout the week.
With the rise of television, traditional radio stations with their blocks of programming started to collapse, and radio had to reinvent itself in order to stay in business. Thus began the age of America's 24/7 music formats on AM radio, and later on FM. They were oftentimes brilliantly programmed, and customized for their markets.
Fast forward to the present time, 2013, and we're at the age when the global internet revolution has dramatically altered the landscape.
Listeners have access to radio stations, not just from across the country, but from across the world. So listeners from Des Moines might be tuned into New York's Z-100, Capital FM from the UK, a net-only station like .977, as much as they might be tuned into a local Top 40 outlet (107.5 Kiss FM in Des Moines case).
This in turn may encourage some radio companies to re-evaluate the need for local radio.
While I personally love local Top 40 radio, there are also some great regional as well as National Top 40 chains I listen to from around the world
Examples of national Top 40 stations:
BBC Radio 1 UK
Jovem Pan Brazil
NRJ France
Evropa 2 Czech Republic
Los 40 Principales in Spain and all over LATAM
Europa Plus Russia (a country with 11 time zones!)
Radio Deejay Italy
ZM New Zealand
5 FM South Africa
Cidade FM Portugal
Virgin Radio Turkey
Example of regional Top 40 stations:
Radio Energy Quebec Region of Canada
Flaix Bac - Barcelona/Catalonia region of Spain
Radio Contact Brussels/Wallonia region of Belgium
Beat 102-103 Ireland
Viva FM Milano/Lombardia region of Italy
Eins Live Cologne-Essen/North West Rhine Region of Germany
You FM Frankfurt/Hessen region of Germany
So, with successful regional and national music stations already in existence elsewhere, and with the availability of streaming internet radio, the future may lead to more of both, alongside locally-programmed music stations
Lastly, has anyone compared the playlists of 97.1 Amp Radio L.A. with that of nearby Energy 103.7 San Diego? Both are owned by CBS, and both are in Southern California:
http://www.allaccess.com/mediabase/q/report/playlist/station/KAMP-FM
http://www.allaccess.com/mediabase/q/report/playlist/station/KEGY-FM