Instant of Eternal
Tethers are rare, and they are important.
There are a few well known Tethers, and they are. Very. Well. Defended. By the same token, they are also very closely watched by the other side (or sides, as the case might be). It’s hard to be subtle when using these well-known Tethers, so they are primarily used for when the action is supposed to be public (or as public as the War ever gets, these days).
The best protection for a smaller Tether is secrecy and silence. No one will destroy, redeem, or corrupt a Tether if they don’t know it exists.
A Tether provides essence for the Superior whose Word it serves. Utilizing a Tether basically cuts off the supply lines for a while. If the Celestial doing so had good reason, no one will complain. If it didn’t have good reason…
A Tether functions less like a highway, and more like a lighthouse, or in the case of strong Tethers, a series of trail markers. The path is neither quick, nor easy. It is however, quicker and easier than the other option — walking through the Marches.