> Agile gives the programmer a voice in the process

This is not my experience of "agile"; in practice, it's a conveyor belt of user stories in a feature factory, and developers are simply cogs in the meat grinder.

You can try and feed information in via estimates, sure, but that can only marginally change how the product evolves under the prevailing winds. Unless you give silly estimates, you have a hard time changing product management's mind much. And it's not a route for coming up with new features by recombining already existing work.