From: Craig Berry

> Spin stabilization doesn't work the same way in vacuum. The in-atmosphere version is designed to combat drag torque effects to keep the
> streamlined profile nose-on to the direction of travel. Drag torque doesn't happen in a vacuum. For a dumb projectile, orientation won't
> matter at all on impact. A smart projectile will need active attitude control (gyros, jets, or a combination.

While it's true that there's no atmospheric effects for spin stabilization to counter, it's still something that can counter tumbling due to uneven forces on exiting the barrel. It's also a good way of stripping a sabot, if you have one. The drawback being that adding spin has costs -- the reason we use smoothbore isn't because fins are better than spin, it's because a smooth bore has lower friction and therefore gets higher muzzle velocity.