So, I see a lot of players using "critical stack". I have to say, I don't agree at all. Here's why:
Most of the time the critical ability I am observing hovers between 12% and 24%. Let me break it down mathematically: The units only get to roll that the first time they attack. So, the number of times that ability is used is dependent on the number of units you lose. So if you have a decent stack, you're looking at somewhere between a 10% and 50% chance of killing a single unit in a battle. One unit. If everything but your hero dies, allowing 8 rolls at 12%, you've still got a 35% chance of critical hits never landing. About the only time this can be offensively useful is if grouped with Ghosts and/or fodder units with an existing critical stat. It could be considered defensively deadly if it's high enough (perhaps grouped with Ghosts) and the enemy must cycle through multiple stacks of fodder units when attacking a city. Rare circumstances indeed.
Compare negative stack: Expensive, yes. But it comes into play every time you kill an enemy unit. It keeps your own units alive. It also can't be directly countered.
Command: As with negative stack, your units have an advantage of 1 every time they fight (in the absence of an Archon, and noting that reducing the enemy strength is more powerful dice-wise than increasing your own by an equal amount). This is significantly more useful than even a 30% chance of instantly killing the first enemy your unit fights. It comes into play every time your units are matched up, not just the first round of the first matchup for each unit.
Stack+terrain/attack/defend: Situationally useful, and not nearly as good, as these abilities don't stack with other stack bonuses. However, in a vital battle, it can often be extremely effective to drop 30-40 points to get +2 when attacking an enemy hero stack or city at a critical moment in the game. I have found the reward in hero points to often exceed the investment by a large margin.
So, in short: Critical stack is (relatively speaking) the most useless group ability there is for a hero. Why do so many people love it so much? Do they just not know how it works? Are they copying each other without thinking it through? Or am I totally wrong, missing some vital factor?