I personally think that the art restriction for crucible can be lifted without any problems.
As some have mentioned, crucible ideas are still relatively raw and unfinished.
Moreover, as an artist, it is frustrating to spend a lot of time and effort making a good art concept only too see it go to waste if the card doesn't advance.
I do think art should begin at the forge level though. This is the gateway to Cygnia, where the card actually comes to life and people get to see it in action. As such there should be some form of artwork. If nothing else, it helps distinguish the card from others.
From a playing perspective, it gets frustrating when there are too many artless cards because, being visual creatures, humans can recognize a card more easily from a picture than from having to read the name.
If a card has art going into crucible, though, all the better. It does help to bring the card to life and can help shape its personality, theme, and mechanics a little better than just a name alone.