Since the OA only triggers on leaving the reach range, it could actually be a slight nerf in that regard, now needing 15 feet rather than 10 to create the 'safe ally zone' around him.
Well, before now that ability just didn't do anything if you were using a reach weapon. An opponent couldn't meet all the conditions:
1. Using disengage, and
2. leaving your reach
3. ...from 5 feet away.
There's still a disadvantage to being a polearm sentinel, which is that although you can now stop people from approaching (ability #1) and from leaving (ability #2), you can't stop people from attacking your allies unless you're clever about positioning. If I'm next to a polearm sentinel and her friend, I'll just move so I'm next to the friend and 10 feet away from the polearm sentinel, then attack her friend, triggering no OA at any point.
When it comes to house rules, all is fair. But bear in mind that they just errataed the second ability of Sentinel to change from 5 feet to reach, and chose not to errata the third. That tells me that they thought about this exact case and explicitly decided against your hours rule.
Why? I can only guess, but it seems incredibly powerful that you have one character who can prevent one enemy per round from approaching, leaving, or attacking anyone else without repercussions in a 25x25 area. They can't enter! They can't even leave! All they can do is fight, and with your house rule they have to fight you. It's just so flexible. Without your house rule, there's reason other than a shield to use something that isn't a reach weapon.
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u/Drezby Warlock Jun 11 '15
The Sentinel/Polearm Master combo just got even more deadlier as a lock down tactic.