Dying because you cannot control your character is always aggravating, and getting a kill because your opponent was stunned is never satisfying (same goes for knockdown).
That being said, the polearm left swing would have to be buffed to compensate for losing stun (seriously - it's nearly worthless). In addition, the spears would require an increase in side-swing damage.
Edit:
The support polearms (long spear, english bill, awlpike, etc) absolutely should not have polestun. They are already among the easiest weapons to use in group combat. The easier something is to use, the less powerful it should be.