Well, most of cases you will have layouts which are, or will become, complex, and it will be difficult to calculate the positions programmatically.
And it will be also a disadvantage mantaining it, because you will not be able to use the interface stuff (grafic layout and so on), and other people, or yourself, will not understand the calculations the same way they would if they see the views in XML. Reorganizing, changing somewhere a position could be painful.
You also will be working frequently with bitmaps, which have a fixed size, if you calculate the dimensions programmatically and stretch they will not look good. At least you would need different set of bitmaps and load accordingly.
It helps if you for example use relative layouts with rules (like above of / align at the bottom of the parent, etc), linear layouts with weights, and dip (density independent pixels). You should do programmatic layout only when it's not possible in other way. Or in some certain cases where it really-really makes things easier.