It's only worth it if there is some sort of benefit to achieving silver that you think will improve your trip. If you look at the benefits list and can't see any that would actually make your trip any better, then there is no other extra "hidden" benefits to speak of, at least for Silver.

This is particularly so if you aren't even flying with Air NZ and are just flying with Star Alliance carriers for your RTW trip. The only thing they care about is whether you have Gold status, because that in-turn gets you Star Alliance Gold, which is when the best SA benefit comes online - lounge access.