I don't doubt what Ian says, but it just seems interesting that this article
says just the opposite.
It's about car audio, but the theory ought to be the same. Under ADVANTAGES this is what it says concerning off-axis.
"Greatly improved horizontal off-axis response resulting in excellent high frequency detail retrieval, a more open and "airy" sound, and a wider/deeper soundstage."
Edit: But under DISADVANTAGES it also has this to say, that I just missed until now.
"Limited vertical dispersion. While this can be an advantage, it can also be a disadvantage. The tweeter must be aimed directly at the listening position, otherwise the spl will drop off. Can be a pain to work with if there are 2 people of different height sitting in the car."
So, good horrizontal, but poor vertical dispersion. It would then seem like both a dome and a ribbon would be the best of both worlds.