Lots of great replies, thanks guys. I think some of the "off" replies are based on the fact that the term "songwriter" is very vague.
Songwriters come in various shapes & forms. Many do not play any instrument with any proficiency whatsoever. For them, loops are invaluable. The ability to change a loops tempo & key with ease would be indispensable. As I recall, Sonar is very good at that (my last version was Sonar 5).
Some songwriters can play an instrument, but they aren't "very good" for them, the ability to correct tempo of a recorded clip is very useful. For the person who plays keyed instruments (Keyboards, synths, piano, etc...) midi quantization is a great tool. Input quantization would be helpful. For the hack guitar player (raises hand) the ability to quantize audio would be great, the ability to slice out a note/chord & raise/lower it's pitch would be great.
As mentioned earlier, loops are a great way to organize & compose an arrangement. Do I need two choruses? What happens if I repeat the Intro here? etc... Being able to move them around freely is very helpful. & the Matrix in Sonar X2a looks like a good way to do that. Can I record my parts directly into the matrix like I can with Live?
Other DAW software includes this functionality as well. Some implement these better, some worse. The same can be said for many of the other issues some of you have mentioned that I did not, like staff view, publishing, virtual instruments, session drummer (which I agree is a great help to a non-drumming songwriter)
After Sonar 5, I haven't bought another DAW (until recently) because I realized it wasn't really what I needed. It was more geared towards the producer, for mixing & editing purposes. If you had a song that needed to be recorded, mixed, & produced, then yeah Sonar performed very well. Then Ableton live came out & I think changed the whole industry.
I didn't buy Live, but to me it seems like a lot of the concepts introduced by live has been adopted by other distributions. Cubase & ProTools developed play lists, where you could name a section of a song "Intro" or "Verse 1" then tell the DAW which sections to play & when. Which I thought was a major step for the songwriter.
Sure you could do the same thing with Sonar, with a few mouse clicks, but it was a "manual" tedious process compared to the way you could do it in other DAWs.
Recently, I bought ElevenRack, a guitar processor/audio interface for ProTools. it came with a free copy of ProTools 10 (basically LE)so I got to thinking about DAWS again; basically looking for something that can improve my current process, or just make it easier.
When I got away from Sonar5, I just started working with hardware; either a Fantom-S, Fantom-X, or an MV-8800. I sample a guitar loop, build a pattern around it, then arrange it. After I have a "working copy" I track it from start to finish into an AW4416 (another reason for looking into computer based DAWs, is that the AW is about to give up the ghost). My daughter plays the drums, I play guitar & bass, I record the sequenced keys (if there are any), then I'll sing or my daughter would sing, or we'll both sing.
Then I'd edit, mix, apply effects, & master all on the AW4416. Which I can do with Sonar 5.
So my current need isn't to replace the AW4416. I want to replace the MV-8800.