"Now, if purple gear was fully moddable, players would simply farm the first boss, acquire 5 pair of boots, extract the armoring, mod and enhancements from the boots and slots them in their favorite Custom items. That actually sounds cool, but it really isn’t. Letting players extract the armoring mod from the purple items would trivialize end game gear progression and stop rewarding successful and dedicated players for their efforts."
http://www.swtor.com/blog/some-clarifications-item-modifications