What's your specific need that you think landscape will fulfil?
The reason it's done in portrait is so that it's the maximum resolution possible.
e.g. Say a panorama is 4:1 (4 times wider than it is tall). The sensor on the iPhone in landscape is 4:3. If you add two landscapes together, that's 8:3. Add a third, that's 12:3, or 4:1. So you can think of the panorama as made of 3 8 megapixel photos (if iPhone 5). Now if it was portrait, the sensor is 3:4. 2 pics side by side = 6:4, 3 = 9:4, 4 =12:4, 5 = 15:4. A little bit more will make 16:4 which is 4:1. So a panorama of the same aspect ratio is made of 3 photos if done in landscape and 5 and a bit photos if done portrait. The latter method has more detail.