Not in 5th edition—if you want to make AoOs against enemies who close into your reach, you need the Polearm Master feat.
Also, reach weapons tend to deal 1 point less damage, on average, than two-handed swords and axes.
Who would ever build a character concentrating on Pole Arms and not take this feat?
And, at least until the late game, a character who chooses the feat will be at -2 to Strength compared to the obvious alternative, so you're really comparing a "free" opportunity attack to +1 to hit and +2 damage.
Who would ever build a character concentrating on Pole Arms and not take this feat?
Feats are optional rule. The game assumes people are not using them. My group does not use feats.
Yes but few people like me who build characters for a game without feats would use polearms, greatsword does more damage, and meshes with great weapon fighting style better.
So to answer "Who would ever build a character concentrating on Pole Arms and not take this feat?", someone who didn't mind doing less damage for the sake of the flavor of having a polearm, and for the few times fighting with reach is important, in a campaign that didn't allow feats. So almost no one ever.