PM's challenges with Agile go beyond how to interact with development in "real time" - they now have to translate what comes out of development to the rest of the organization. Odds are the rest of the organization isn't operating at the same speed that development is.
What does this mean?
Exampli gratia: development has decided it wants to use an Agile methodology and throws all its old waterfall chops out the window. The PM translates market requirements so development can make use of them, writes stories, owns the backlog, sets the priorities, these are pretty well known PM activities.
But now you've got "stuff" flowing out of development much faster than in the past. Marketing and sales need to be able to make use of this "stuff" - marketing needs to understand it so it can position it, sales needs to understand it so it can sell it.
Odds are strong that no one told sales and marketing that they need to be agile. They're in their own balkanized silos, they dance to their own tunes, they have their own bosses. They probably look at development with the sort of fixed smiles you save for "special people". They're not built to do sales training every few weeks to spin sales up on new capabilities. Marketing is not built to refresh the website, collateral, partners, press, analysts every few weeks.
And don't forget all the "stuff" zooming out of development needs to be rationalized to a pre-determined strategy, fit in to a pre-determined roadmap, and be able to demonstrate value through whatever success metrics have been (wait for it. . .) pre-determined to be important.
That's a lot of documentation to update. But you need to update it all because documents matter - if you're not writing all of this down, it's not real.
Agile puts the PM into the role of a "transformer", stepping down 220 volt output from the development plug to the 110 volt sales and marketing engine. Without a transformer, whatever you've plugged in will burn hot and bright for a while. . .then burn out.
In summary: just because development can create it, test it and ship it fast doesn't mean the people who need to consume it are ready for it. Ever have a waiter bring your main dish before you're done with your appetizer? Remember that feeling.
Unless your organization is totally harmonized around the idea of fast product turns, the product manager needs to "act agile" with development but "act waterfall" with sales and marketing. PMs who create "product harmony" in this way allow each silo to operate the way it wants/needs to operate to produce optimal results. That's a formula for success.
I'm curious what your real-world experiences are in this area.