Like I said, it would be an option for those who want to use it.
If it was optional, I'm not sure how they would manage it. The moment XMR comes in, where will it be enabled first? On the regular exchange or the orders on the list will have priority?
The concept makes sense, but it looks more like a p2p model and I wouldn't add it to instant exchangers.
If there are no reserves for the coin I want, the exchange has to send back my deposit. I then have to deposit a second or third time if I am unlucky. That's throwing away money on fees.
The same thing could happen if you are on the waiting list for too long and you want to give up and withdraw your funds.