I just did a few back to back stouts and would agree that pH is the main thing. If you mash only the pale malts at reasonable pH then add the dark grains you'll drop the pH real low. I did that intentionally to get some of that Guinness character - the final beer ended up at 3.85pH and that was a big factor in it's taster. Adding all the grains in the mash and mashing higher (5.5-5.6ish - final beer was 4.2 IIRC) resulted in smoother beer for me.
If my goal was really, really smooth dark beer I'd go with dehusked black malt, Carffa III special, black prinz, midnight wheat, etc. Did an oatmeal stout with all the dark grains dehusked, it added a great smooth chocolate flavor.
This is one thing I've always wondered about this method, is dropping that pH too low after adding the dark grains at vorlauf. I subscribe to JZ's thinking in that brewers have been mashing grains all together for centuries, no reason to change it. I doubt Fuller's is adding their dark grains to the London Porter at vorlauf...
I'm using debittered black in an oatmeal stout today, I'm really looking forward to it.