That still wouldn't solve the problem of people who pathalogically must open every spoiler. They couldn't stop themselves opening one spoiler and then there was nothing but a spoiler in it, labelled "really spoilery honest!" So, are they not going to open that one since the empty outer-spoiler sated their curiosity?
So is there going to be a heirarchy of spoileryness that posters must adhere to in a nested fashion. What if you opened a level-1 spoiler but the contents were clearly of a level-5 spoilery nature? You were lulled into a sense of security that spoilers didn't actually contain spoilery material, then you open one and - bang - entire end of a major series blown. If we had a convention with nested spoilers and level 1 spoilers were "safe enough" that people just opened them anyway, then it just moves the problem along while making the site harder to read.
Personally, I think rather than nested spoilers we need an alternate "Block" collapser which is for collapsed material that isn't spoilers, such as collapsing down images or walls of text, but indicating that they're not actually a spoiler. Perhaps, add some code such that automatically wraps images in a block if you spam a lot, or big, images? That would actually be pretty cool since it would be an anti-troll feature, as well as preventing newbs from posting too many images, and save time when you're trying to actually provide a gallery of images.