Its just much more fluid for me to spill out my mind onto paper than diagrams or UI layouts on the computer.
I'm sure one day when you can draw onto a tablet just like putting pencil to paper I will switch, but that day is a long way off.
A Cintiq is 5% the way there. And digital pens for tablets are as far away as a mouse.
Any lag, any lack or precision and also the back light are all issues. I am confident they will one day be solved and I will carry a true digital notebook where the contact of pen to paper is indistinguishable from the real thing.
Why do I long for this future? Because I love digital and I wouldn't all my work on paper being stored as a digital notepad. (Scanning is not a solution because I want to pull up notebooks from the past and work on them using my "digital notebook")
This was a bit a pointless post actually, Instead of deleting it I'll leave it here in-case anyone has anything to add or disagree with.
Before settling on a tablet, I considered the Wacom Inkling and even tried using the iPad and a few apps, but could never quite get the same level of precision as I can manage with a pencil. The tablet's just a stop gap, and keeps me from going crazy and loading up a boatload of sketching gear (psst… Copic pens). My Intuos 5 + Autodesk Sketchbook Pro combination gives me some degree of freedom without the clutter of gear I'd unavoidably end up with if I went shopping for stationery.