Its been on our radar for about 10 years :) In fact we started some prototype work on this as far back as 2006.
We all know it would be a useful feature to have but we have had other priorities and it would have taken quite a large investment since it does require some retooling of the engine for seamless playback of disjoint regions. Not to say that we won't do it but its not yet in the pipeline. Doing the basics for non linear arrangement are not that difficult but getting streaming to work flawlessly is a different matter. The matrix playback engine essentially does non linear playback but there are some caveats that would have to be designed for.
Actually there is a question about primary usage that would be interesting to know. Are users expecting this to be a tool for arrangement primarily or a live playback tool. If seamless realtime playback of sections is secondary then this could be a simpler project. i.e. if you can layout the song, preview it and then and render it as a wave file would that meet the main requirements?