I'm not trying to knock anyone's method. But, an iPad for portable songwriting is the greatest thing since sliced bread. With it, I can get my ideas down wherever I'm at. Then, when I get home, transfer them into Sonar and finish them out.
Does everything have to be a full featured recording studio in a box to be useful? I think we've become too dependent on having a gazillion tracks, the latest and greatest plugins, and whatever else our wallets can afford us. We are spoiled. We shouldn't need all of that to write a good song. Just something we can bang out some notes or chords with and anything that we can scribble down some lyrics on. That's it.
Enter the iPad.
It's got everything you need to write a great song. Right there, in the palm of your hand. Want to add guitar? Great, plug it in, get to strumin'. Lay down a quick dummy vocal so you won't forget your melody and lyrics. Push record and go for it.
Not being able to write a song on an iPad because it doesn't have pro-level inputs, outputs, amp sims, VST's, etc ... is like saying you can't drive to work in a Fiat because it's not a Mercedes. It's the same principle.
Capture the idea down while it's hot. Pull the car over and write that hit record while the vibe is knocking you in your head. You know you'll forget it by the time you get home, talk to wife, deal with the kids and pet the dog. That's what an iPad excels at. Getting the bones of that song's body written. You can get technical and produce it all fancy-like later.