Read why I explained audiotracks are a bad idea. The same is true for a piano roll.
You are forgetting about the amount of work needed to add fundamental changes to Renoise’s feature set. Not only do they have to code the thing,
they also have to keep supporting it (debugging, adding any features people request related to it). Adding a piano roll is opening a can of worms (or bugs, technically). It is a prime example of feature creep ( http://en.wikipedia.org/wiki/Feature_creep ).
Your viewpoint is exactly what leads to software bloat: http://en.wikipedia.org/wiki/Software_bloat
The idea that unused features are easily ignored is delusional. One only needs to compare an Apple product to a Microsoft product to see this.
I’m not even sure the development team is large enough to implement these things rigorously (or affordably).
Look, the developers have spent the time to implement ReWire so you can add another DAW for these features. They did this, I’m assuming, because they don’t want to make Renoise a vertical Cubase, or vertical Logic, or vertical FL Studio, or vertical Sonar, or vertical any of the other hundreds of DAWS that are designed for those things.