Ok, this is how we should look at impale for tanking.
Your talent spec with 7 points left is this:
Though if you like the Imp Bloodrage and Imp Shield Wall could be other low level prot talents.
You can do one of these things with those 7 points:
3 Deep wounds + 2 Impale + 2 Cruelty
3 Deep Wounds + 2 Impale + 2 Vitality
4 Cruelty + 3 more in a combination of Demo Shout / more prot talents
So the question is which of these options is best?
The cruelty prot option gives:
4% crit (4% dps) + some utility/mitigation
How valuable is Deep Wounds+Impale?
2/2 Impale increases your dps by a percentage equal to 20% of your crit rate, for special attacks only (including heroic strike'd autoattacks).
With a heroic strike on most of the autoattacks, your going to get around 80%+ damage from specials, and the rest white damage. Best way to find this is to use a damage meter on the boss fight, and see what %age of your damage is from melee. Lets say its 80% specials.
If you have a 20% buffed crit rate vs a 73 mob, and 80% damage from specials, then 2/2 impale will give as much dps as 3.2% crit. At a 10% crit rate, its worth as much as 1.6% crit.
Now for deep wounds: With a king's defender this will be 140 damage over 12 seconds. Thats 11.7 dps. This will be up most of the time with 20% crit rate buffed, and doing attacks every 1.6 sec and specials every GCD. If its up 75%-80% of the time, thats about 9 dps. Thats about a 2-3% increase (against things that arent immune). I dont feel that the deep wounds debuff is at all a bad thing. You shouldnt be attacking a CC target mob. If you do, the impale build clearly isnt for you. We'll say deep wounds is a 2% dps boost to be conservative.
So with 20% crit, we get 3.6% off impale and 2% off deep wounds. Thats a 5.6% dps increase, or more than we'd get from the 4 cruelty and 3 marginal talents. But if your crit rate is only in the 10-15% area, its not going to be better. And remember tha this is with only 1 rank in cruelty.
We also have the option of moving 2 vitality into cruelty, getting 3/5 ruelty and 3/5 vitality.
Basically at that point you have to choose between 1% crit or 2%str/1%stam. I think the str+stam is better, mostly because you need the stam.