There's only a narrow set of circumstances in which that's better than Mobile, and a broad range of circumstances under which it's worse.
If the enemy starts 10' away, the bugbear rapier rogue can stab him from 10' away and then move 30' and Hide. The Mobile human rogue can move 5' toward the enemy, stab him from 5' away, and then move 35' and Hide. Same result.
If the enemy starts 5' away, the bugbear rapier rogue is in trouble. He can stab him from 5' away, then Cunning Action: Disengage and move 30' away, but the enemy can just pursue him and attack again. The Mobile human can stab the enemy from 5' away and then move 40' away.
It's pretty hard to come up with a corner case where the Bugbear is better. (One example: three Tiny enemies in the same 5' square 10' away at the start of the turn. The Bugbear Rogue can attack and fall back without being attacked back; the Mobile human will get attacked even if he does an offhand attack. But that is an extreme corner case and relies on exact positioning at the start of the turn.)
What is the actual scenario where an extra 5' of reach is better than an extra 10' of movement, ability to ignore difficult terrain when Dashing, and immunity to opportunity attacks from creatures you've attacked? How do you leverage that 15' reach whip into a tactical advantage instead of just a capability?
Because Sentinel. That and you're a defender, you don't want to be moving. In a similar vein, Polearm Master + Sentinel is great on a Bugbear.