[GingisK]

My take is that the catching arms is what makes the fins unnecessary.

The fins are used to steer the booster into position for the catch. Steering is a combined effort of using the gimbaled engines and the fins. Engine firings are limited by fuel reserves, the fins are "free".

[Carry_Okie]

The fins are used to steer the booster into position for the catch.

Duh. I was talking about the traditional fin design.