At some point, Lanfear notes that Rhuidean was shielded (she might've used the word warded) in a way that she couldn't figure out. Also, that Asmo + Rand fighting dispersed it in a way so that she couldn't puzzle it out.

As to alternative uses, Mesaana herself notes that a Dreamspike would serve to protect her dreams even better than wards would. That's pretty handy...

Also, off the top of my own head - a dreamspike is extra-useful because it only allows the traveling that you permit. It's like they figured out public/private key encryption, then applied it to instantaneous Travel. This allows you to create an area as a staging ground for transporting troops.

And, of course, as we saw in ToM, a Dreamspike can be used to set up a pretty good trap. Imagine if you knew the exact circumference of the effect, then made weaves as traps all around it (or within it). Suddenly, even people who know they have to move in order to Travel can just run right into invisible ribbons of air. Or explosive gravel.

That's all I've got, at the moment...