After extensive testing on my own, I've figured out that there IS a real reason for skipping wallpaper. I have an iPod Touch G2 (that I connect to my ThinkPad, before you get the idea I'm an Apple nut) and with the update to OS 4, performance is certainly very acceptable. However, as noted, it lacks the wallpaper and multitasking capabilities. So, I used RedSn0w to JB and get those features. The end result is that, although performance is just fine most of the time, there's a severe lag when I open an app folder on the home screen. The iPod pauses for a moment, and the frame rate is somewhat low for the duration of the folder-opening animation.
When on a plain back wallpaper, this isn't the case. I suspect that this may have to do with rendering transparency for the icons that are not in the folder opened (since the transparency for a black background only involves darkening and reducing saturation).