Page Layout forces you to work in code-mode all of the time. There are times when design mode saves me time with naming controls , finding things easier, etc... With the page layout in design mode you can't do anything but look at a bunch of garbage and you can see any controls. Maybe that is tied to ASP.Net master pages in my case.
Device detection could be enhanced if for say I am using the Chrome developer tool and emulating an iPhone 6 it could recognize that. However it only (seems) to know the computer monitor resolution I am developing in so it is always detecting the largest resolution.