No, all box arts have some element of the plot involved! For example, if you looked at 358/2 days box art you could see a couple of plot pnts! The fading/vanishing background symbolizes a couple of things. First.....the fading/vanishing background can either represent how the story of that time was forgotten by every1 or how xion faded away from everybodies memories........Well that is the same thing.
And also from by looking at the box art from left to right.....it tells you the story somewhat. Everything at the left is before roxas was born hence the whiteness. And when you go to the right you see differents pnts of the story. Roxas meeting Xion......Hanging out with Axel.......Meeting with Xion at the clock tower 4 the last time. And as well as meeting/battling riku.......and him w/ the digital twilight town(the computer that controls the program)
It shows the palm tree and a bush that you see on destiny island, the clock tower you see in twilight town, the skyscraper in the world that never was, and the five characters this game is mostly around. Now I don't see how this cover art is spoiler-ish, it has the characters that are in the game and shows some of the worlds. Now im not bashing on your statement but honestly I don't see how this cover art gave any of the game way. I mean look at the cover of Kingdom Hearts, they didnt even use that world and that world was the world that never was, they used that in Kingdom Hearts 2, now would you say that cover spoiled the second or first game, probably not, again I'm not bashing on your statement. I'm just saying I don't see how this cover art or the cover art of Dream Drop Distance gives any spoilers, grant it I haven't played Dream Drop Distance yet but again peole are just going off on speculationgs about oh light falls into darkness something is gonna happen to sora where he will fail the test or go to the darkness. Thats why I don't like to speculate on a game, granted I have done it but I try not to. Anyway, I'm just saying that as far as I know the cover dosen't give a spoiler or anything, it just looks firetrucking awesome.