I did the math and found that impale does not increase your tps by a significant amount. In fact, you will only come out even from choosing that over the 2 points in cruelty if you have 7% crit before cruelty and even then only if you are rage capped. Just for a 1% tps increase with impale you will need 19.5% crit which is unfeasable for tanking.
CritThreat%bonus = 0.4 * (BaseCrit + PointsinCruelty) * (0.1PointsinImpale + 1)
Let I(C) be a function that represents the bonus threat percentage from impale spec dependant on your crit before cruelty, or C. Let S(C) be a function that represents the bonus threat percentage from the standard 5/5 cruelty spec.
I(C) = 0.4 * (C + 3) * 1.2 = 0.48C + 1.44
S(C) = 0.4 * (C + 5) = 0.4C + 2
I(C0%) = S(C0%)
0.48C0% + 1.44 = 0.4C0% + 2
0.08C0% = 0.56
C0% = 7
I(C1%) = S(C1%) + 1
0.48C1% + 1.44 = 0.4C1% + 3
0.08C1% = 1.56
C1% = 19.5
Is there something I'm missing here about impale because it seems like every MT of an 8/8 BwL guild has it?