I don't want to answer this question, because I will end up writing a 500-word essay =p but let me try to be brief:
On policy: I tend to defer to Drocket, who is the only active admin to my knowledge. As far as I'm aware, wiki editors in general accept that "clutter" will be added--it's better to have it than not (otherwise, how are we that different from Steam Card Exchange).
On my adding those backgrounds: I actually try to avoid updating the Sales pages because they always have extra stuff, but lots of those images were sitting in Special:UncategorizedFiles for a LONG time, so I just finished up the page as best I could to have those finished. The other obscured backgrounds were already added, and I dislike deleting any work other people have done, because it clearly means something to THEM (what if someone turned an obscured background into gems, then wondered afterward what it turned out to be?).
My general opinion tends to be - I dunno. I can't imagine any real use for them either. At the same time, what if someday we DO want them for some insane reason - I don't think we could get them back, since they're a limited time sort of thing. I guess it's better to have them and not need them than to need them and not have them? Of course, that's the sort of hoarder mentality that's the reason my house is a mess :P