I'm interested in how they could be used, also. I'm also interested in whether or not it could have something to do with the veil rifts. Some people theorized before that the Eluvians could have something to do with them. This definitely lends a bit more credit to the theory that they're involved. I remember someone specifically pitching the idea that someone was manipulating the elves into using the Eluvians just a few days ago. Looks like maybe they had it.
I would think the main problem with the elves using them as an invasion method is that it sort of hinges on the protecting all the gates, right? I mean, if they lose a gate, then the opposition would be able to reach them beyond the fade and at their other gates, right? Seems like a risky strategy. They'd only be as strong as their weakest link, basically.
The control network aspect of the central eluvian implies that you can just use an eluvian once you find it- other conditions and such must be met as well. Then there's also the point that the Fade Tears/invasion may simply be a byproduct of something else: either a distraction to a different goal (if you believe the rumors that the true instigator is using them as a distraction while it breaks into the Black City), or a consequence/means to something else. Invasion and conquest doesn't have to be the goal, after all.
I suspect that the Dalish Dreamer's fade boss is the source behind the Veil tears, but I'm torn on whether I should suspect the Eluvians to be the tear points or not. Not all of them, per say, but rather that the eluvian network gets corrupted such that eluvians not secured/in use (such as those used by the City elves) could be corrupted/turned into tears in the fade.
On one hand, it makes some sense both in terms of set-up, causation, and loosely defined mechanics. The Fade Boss clearly has knowledge, influence, and manipulation at play, and so the activation and subsequent corruption of the Eluvian network is an indirect but reasonable outcome of its efforts: a process that it could initiate and expect to be fullfilled when the time is right (as it is now). The scattering of the Eluvians... might not explain the tear in the sky, per say, but could explain other tears: if certain eluvians could be made into tears, then long-forgotten or inaccessible eluvians (like, say, at the bottom of a lake) could explain the placement of some tears. Others, like in the sky, might be a reflection of the disturbance points. And to round it up, the idea that the eluvians are already weak points in the material/magical realm make them logical weak points of the Veil to start a tear from and to re-seal it at.
(I might have had a little of The Order of the Stick and the snarl tears on my mind when I thought that.)
It also helps a general 'status quo is god' aspect of the setting if something as potentially game-changing as the Eluvians were rolled up and destroyed/sealed in a short period of time. The Eluvians trigger great impacts before they are sealed once again, etc. etc.
Plus, if the Eluvians are a part of the problem, then ancient elven knoweldge would logically be a part of the solution. Hence visiting Arlathan.
On the other hand... well, not much else but that it seems a bit obvious. Which doesn't mean the eluvians being fade tears in the waiting is necessarily correct, or that there couldn't be big ass caveats to it, but it seems a reasonable sort of speculation so long as it is remembered as such.