I don't think it really matters.

I think it's just a poor argument for them. The wheel slipping? Track bikes were steel before they were anything else.

I'd just say that it isn't necessary. The paint doesnt get chewed up on the inside, cause there isn't anything rotating. On the outside, the axle twists against the metal, messing up paint. On the inside it doesn't.

But if you're gonna do inserts, why wouldn't you plate both sides?