Speak with Dead can have some skill challenge tech if needed and I'd imagine it would be pretty simple to bolt on skill challenges to many other rituals such as Shadow Walk, Tenser's Floating Disk (for people who just *have* to ride it), View Location, and so on. I can't speak for my fellow designers, but I know I'm reluctant to include too much skill challenge tech b/c the caster already has to invest components for the ritual to take effect. Adding additional risk for failure, while cool sometimes, can be discouraging.
Something I'm kicking around for my own games is to use skill challenge mechanics to perform rituals without the Ritual Caster feat or without components (or components that don't have gp value--eye of newt, virgin's tongue, stuff like that). Since there's no investment of material wealth, I'd be more comfortable doing bad things to the PCs for a failed challenge.
With Adjure, I wanted to bring back stuff like cacodemon and aerial servant. Since you're exerting your influence over another critter, the skill challenge to cover the bargain made sense and the risk of failure (being commanded) was just the sort of nasty a greedy PC should face when trafficking with things from the "lower" planes.
Did somebody say beer?
