This rune needs some serious changes. I understand that it also gives Elementalist the ability to stack might. But when you compare this rune with other Elite Specialization runes, this rune in particular has very little synergy with the Deadeye's playstyle.
Currently, the sixth bonus from this rune gives you 2 stacks of might for 15 seconds for the use of a Cantrip. Or about 60 power for 15 seconds per cantrip. By definition, Cantrips are designed to be defensive spells, but can be used aggressively. However, using them early means reducing your survivability. Even if you were to use these utilities defensively, you'll quickly realize that there are much easier ways to stack might more effectively than what this rune can do. Let's not forget how inefficient that trade is. Suffer a utility under 20-30 second cool down for 15 seconds of 60 power?
Rune of Rage for example gives you a substantial increase to Fury's duration, and also makes it incredibly easy to get Fury. In Thief you have Critical Strikes, there is a trait that rewards might for every critical hit you land while under the effects of fury, while increasing Fury's duration. This obviously has a massive synergy to how all specs for Thief plays out, and a very powerful one for Deadeye as it allows them to sacrifice a core stat because 20% critical chance is guaranteed to them.
But what if you did not take Critical Strikes? There's still no real reason why you would want to use the Rune of The Deadeye.
Rune of the Holelbrak - You rely on deadeye's natural ability to generate might, increase the amount of time it lasts, while at the same time reducing the time conditions last.
Rune of Strength - +45% Might Duration, 25% chance to gain might from being struck +5% damage under affects of might. Deadeye has a strong ability to naturally generate might, he can have a very long up time for might and also that additional 5% damage.
Rune of the thief - +10% damage from attacking from behind or the side
Rune of the Wurm - Self explanitory.
Rune of the Dragonhunter - Thief has access to traps, and traps do a kitten good job with Deadeye.
My personal Suggestions to make it a much better option for the spec, is to target what the spec can synergize with.