There's more in the update than roads...there's speed limits, businesses, phone numbers, charging stations, potentially improved terrain data (the car uses elevation changes, speed limits, distances, etc. to pick between shortest, fastest, and most efficient routes) and probably other things as well. Throw in potentially reassigned one-way streets, etc., and the updates could be quite useful. It depends on where you travel and how much actually has changed in the area where you are interested in having up to date data.
As said, though, this is only a data update. While the dealer might occasionally have a functional update to the nav processor's software, that is a totally separate thing from the data files that allow it to function. FWIW, I've not heard of a nav program update, but there may have been one along the way since original i3 release. Data updates, OTOH, occur at about every 6-months. Whether it's worthwhile to you to do, can't say.
2011 535i x-drive GT, 2014 i3 BEV