If there is some kind of swap and i proccs while offtanking and you are at 15 stacks EB anyway its kinda mehish.
Even if you get more crits in that 10sec window.
So ya i might use it for when i can make sure i have a high tankuptime. Losing haste and mastery for 10 seconds shouldnt be too bad.
What you cant do tho is mixing this trinket with the Haste stacking trinket tho.
IF it would consider the proc into your haste value you could end up with even less haste when the procc drops during the 10sec window.
Or, if your haste gets high neough the Rune could suddenly double your haste.
I kinda like how it gets more complicated which trinket to chose. I always loved having situational trinkets to switch around for different bosses.