This already happens when you get a duplicate roll (aand you get crafting mats instead)
Letting people disenchant items they won goes down a dangerous addiction road.
Right now the amount you can spend on the gsme is bounded by the amount it costs to end up getting everything. However, if you can disenchant items, someone who is addicted can spend nearly an unlimited amount of money and ruin their life.
I like the system as designed. I just wish you could directly buy crafting mats to make that ine item you really want