We might , after all , want to make our own corrections patches , etc and scripts are editable.
Sure, if we can fit our software for what we want it's a better thing, however why rebuild everything if the work it's already done?
For DE, and even for the iso images, there are some ways:
1) support officially one DE (I hope KDE XD) and do iso image just with it
2) support all the enviroments and make iso for everyone
3) support just 2 or 3 DE and make iso for those environments.
About architectures i think more support there is and better is but we have to think also at the amount of work it involves, so the better thing is find the right equilibrium. How many of us have a i386 machine? How many of us have an x86_64? Nowadays just who do trashware has a machine older than i686...