Easily solved by the polearm fighter by stepping to 10', making attacks, and retreating to 15', forcing the target to enter the threatened space to close the gap once again![]()
In my experience, the monster won't bother approaching the polearm fighter; it will just go attack the mage that the polearm fighter just stepped away from, forcing the polearm fighter to have to engage again. Your mileage may vary, of course.
There are weaknesses to this, of course- limited to 1 reaction, so mobbing the pole weapon fighter limits it's effectiveness. Another is space- if the PA cannot maneuver at all, it steals much of the advantage PA gains (aside from the butt stroke).
Yep, lots of drawbacks that aren't immediately apparent from the description of the feat. The biggest one is related to your 'lack of mobility' comment; a PA/Sentinel character has virtually no ability to exploit his feats while trying to protect a fixed point. In those situations, you'd much rather be a Defense or Protection specialist.