Despite what you might expect, "Max. Number Of Animated Corpses: #2" does not mean that you can have only 2 rallied souls following you at a time. Rather, it means that you can only raise 2 at a time, regardless of the number of corpses in the immediate area. This means that if you are standing on a pile of 5 corpses, utilizing all of them (if this number is at 2) will require 3 castings.
As such, and given that the Rallied Souls' lifespans increase very slowly as a function of combat art level, driving down the regeneration time can be quite valuable, especially when many enemies attack at once.
The target area is really very limited, and is measured from the feet of your character when you cast the spell, so you pretty much need to be standing right on top of the corpses you want to exploit.
Increasing Rallied Souls only affects duration, number of souls and target range. It will not, in and of itself, increase Rallied Souls damage.
Rallied Souls can resurrect anything that leaves a corpse when it dies, even friendly animals or NPC's. This interesting to see but they are useless as minions because they rarely fight.
Pros and Cons
Minions to do the Shadow Warrior's bidding
Can function as killers or simple meat shields
Does not work on Energy type enemies because they leave no corpses
No modifications, no skills, naked, all stats between 25 and 35 (i.e. a nearly-new character).