Focus
I think one of the concerns here is focus.
If you write an item on a piece of paper, no matter whether index card size or poster board size, it is the only thing that you are looking at when it comes to processing.
For me, when I am processing, it actually is more difficult to process if I see 15 other items that I need to process, because I start wondering how they relate. That's why I refuse to process from a list.
Evernote, as mentioned earlier, creates basically, a digital list.
I capture using paper, email, and digitally, but when I process my digital items in Omnifocus, it allows me to use the focus feature so that i can block out all other items in my inbox. That is a big selling point for me.
One piece of paper, one piece of digital information... At least until I reach what I believe to be "black belt".