This is an optional feature that would prevent you from seeing in game chat from anyone NOT on your friend's list.
Assuming my interpretation is sound, I have a concern.
While this idea certainly prevents seeing spam messages (making it inherently good), there are times where I want to be able to communicate with people in public games.
For example, in a large public game I'd like to be to talk to the players I'm grouping with. Whether it's to announce the location of a stairwell, ask if they want a certain item, or simply chat about their builds and strategies, I'd like to leave the lines of communication open.
With your proposal I'd either have to add them all to my friend's list (by manually typing their names, unless I am mistaken) or disable the option and be at risk for spam.
What I like most about the Party Chat feature I've proposed is that you can have the best of both world's. You can prevent spam messages and still be able to easily communicate with players.