why not percentage ?

It is a good question.  I would say you can report process capability in any of the aforementioned forms, and there are numerous tables out there that practicioners use to do just that. Just be consistant in your methods. 
I think the advantage to sigma/Z score (if there is one) is that it is used when dealing with continuous data, where you aren’t counting defects at all, you simply compare the allowed variability   (ie tolerance) to the actual variability (std dev) in your process to determine capability.  Hence, you simply calculate the mean and std dev and predict future capability from there.  And I would think you would need a much smaller sample size to get the same level of precision than if you used an attribute approach.
But it will be interesting to hear the opinion of those more experienced.

Hi Zedd, you got a very valid point there!My take is that the choice of whether it shall be communicated as a
percentage or as a sigma score largely depends on the intended
audience. If your audience is your senior management, for
example, and that their understanding of 6 sigma is just bare
basics, you might wish to stick to your percentages presentation.
You may, as you see fit, indicate a small take-away box at the end
of the slide presentation to explain the % figures when translated
into the 6 sigma world. What really matters is whether you:1 gained the support you need or that a new policy or guidance
was created, reviewed, or repealed,
2 were provided with new or additional resources
3 attained the communication intent or purpose after the presentation. The issue of whether it was expressed as a sigma score or as % a
percentage is another discussion.We communicate to express and not to impress anyway, right?Anton Javier