The map/navigation updates on the Stinger are a paid service. Kia, as have many manufacturers, decided to make the maps an extra-cost discretionary purchase for the car owner, instead of bundling that cost into the price of the car. You are getting a 'free' update so that your new car, as purchased, will be brought up to the latest version at no cost to you. If you had purchased a vehicle with the latest map software already on it, you wouldn't be receiving a 'free' update from Kia.
As for whether to update now, the BT text messaging is the biggest feature in the last update. As that is not something you particularly need right now, then I would wait to update until either 1) you feel a need to update your maps to get improved street coverage, POIs, etc., or 2) another feature is added that you desire to have.
Hopefully Kia will at some point separate the maps/navigation updates out from the head unit software updates, as they are really two separate things. I've been waiting for that to happen since purchasing in the Spring, but haven't seen any movement on that so far ...