I was a 3.x 3PP addict. I had tons (have) of stuff in both PDF and print, and in some cases, the same product in both formats.
For 4e, I don't miss any player option crunch. There's plenty of that in DDI and the CB makes it super easy to use. So unless there's CB integration, player crunch is not gonna be used in my game.
But settings, adventures, locations, encounters? I would buy lots of that stuff up.
Now that there's an import/export function in Monster Builder, there's no reason why a 3PP monster book can't be viable. They can provide a file with their PDF purchase for the monster file. Unfortunately, this file will be pirated ad nauseum and then the value of the book bottoms out unless the fluff is amazing and/or it's directly tied to a new setting.