It is a bit redundant, but it's a compromise between two parties. After you discover all the major cities the carriage system will be obsolete but will still serve as transportation for those who refuse to use Oblivion's fast travel system for roleplay or immersion purposes.
Your explanation was a bit redundant... no offense, but it didn't introduce the actual side of the MW fast-travel lovers: It wasn't for roleplaying or immersion purposes. It was due to the challenge it removed. The fast travel system in Morrowind actually cost you money. It made absolute sense, there are places that if you get to them, like boats, Silt-Striders etc. you can get to other places as well. It's a game, and a world, so fast-travel is a gameplay option as well, when making for free, available from everywhere to anywhere, you actually remove a gameplay feature here.
So no, you didn't really get MW-fast-travel-lovers side.