If you've got a monitor resolution of about 1280x1024, you'll see about 115px of the background image along the left or right side because the content section has a minimum width that can be displayed.
In between 1280x1024 and about 1920x1080, you'll see about 170px of the background image along both sides. The content in the middle will change width depending on the monitor you're using, but the side images will stay at a fixed width of 170px.
It isn't until you have an ultra-wide monitor like a 29" monitor, capable of displaying a horizontal resolution of at least 2400x1000 that you're going to see the full background image along the sides that wikis currently can use that do not have Darwin/Fluid enabled yet.
So I think I've got the full answer to my question now:
- All wikis will have Darwin at some time in the future.
- My 22" monitor can't display a high enough resolution to see the full background image along the side as what used to be available back when the Wikia/Oasis skin was introduced three years ago.
- For special events like what I was thinking of, just stick with a background image of 170px on the left and right side because not many of the kids that will come to the wiki are going to have monitors larger than 25" and run a resolution of 2560x1080 or higher.