the Stalker has high pow Reach, and is the only warpwolf that comes with Pathfinder
Plus he can get Stealth pretty easily, so is easily protected from shooting.
And with his Animus he can run to safety after killing stuff.
The tree is just an all around useful solo. For one, its a Bark-node, so you can sling offensive spells from relative safety. it also denies healing and tough within a small area. Considering how cheap it is, its a no-brainer to have at least 2 of them.
The Stone
UA gives them stealth(meaning your enemy can't just shoot them off first turn) and expands their command range. Allowing them to be much farther apart. he can also throw rocks at people