When designing Apps or web sites for mobile or tablet we have to consider if the layout will change when the alignment of the device changes. I believe this is a kind of responsiveness. Noticed that some apps simply restrict the layout not to change. 

What's the pros and cons of changing the layout besides having to design 2 different versions every time for a feature. 

Will the usability be different from landscape to portrait? 
How should the transition be, when switching between portrait and landscape.