It could very easily be argued that Black and White pull much the same trick with the gym leaders, though. The Striaton trio are obvious. Lenora and her normal types provide an incentive to explore the outer limits of Pinwheel Forest for a Throh, Sawk or Timburr to provide fighting-type coverage, which you wouldn't have otherwise had unless you plumped for Tepig as a starter. Burgh and his bug types slot in nicely before the desert and its plentiful supply of Darumaka, as well as coming at the point of the game where you can get Victini if you fancy giving it an easy run-out.
Elesa has a joint purpose; she comes after the desert, giving you an opportunity to utilize ground types, but she also uses flying types as an attempt to subvert the player's expectations and to encourage a variety of different type distributions and advantages with available rock Pokémon such as Dwebble. Clay leaves an opportunity to use the grass Pokémon you'll find on Route 5 and in Lostlorn Forest, Skyla lets you use the Electric and Rock Pokémon in Chargestone Cave such as Joltik, Tynamo and Boldore, Brycen lets you use a variety of different local Pokemon, such as Klink, Ferrothorn, or Gurdurr, and Drayden/Iris both serve as a preparation for fighting dragons in time for the events at the end of the game, and as an opportunity to use the Cubchoo you can find in Twist Mountain.
There are a few reasons why this doesn't work as well. The Striaton trio are teaching you a lesson you've already learnt. At this point in the game, you've fought Cheren, who uses the starter your starter is weak against, and Cheren's starter knows the relevant SE move (Ember/Vine Whip/Water Gun). If you were just using your starter up to this point, you'd have run into the Striaton trio already, which makes them just a repeat lesson - rather defunct.
Lenora isn't a strong enough leader to force you to vary your coverage. Look at Yellow's Misty. She had a level 22 Starmie. That's base 100 Sp.Atk, 115 Spd, probably a good 6 or 7 levels above what you'll be on reaching Cerulean the first time, firing off STAB'd Bubblebeam which has more base power than any move you'll learn for a fair while. Misty is tough as nails without an Oddish/Bellsprout/Pikachu (unless you picked Bulbasaur). Lenora is not. Herdier and Watchog don't have the same threat level as Starmie, Lenora's team is a lower level, etc. The only real advantage Lenora has is STAB'd Retaliate, but it's only coming off base 85 ATK. You can brute force your through Lenora, which is a lot more difficult for Misty.
By the time you hit Burgh, you've had ample opportunity to pick up a Roggenrola, which deals with him even more effectively than a Darmanitan.
Elesa is well-designed and placed, probably the best of the BW gym-leaders.
For all of the remaining leaders bar Iris/Drayden, you'll have been using Pokemon which can deal with them more than adequately long before you meet them. Drayden/Iris are well-placed again, though, I agree they act as a nice way to introduce Dragons before the end-game.
On top of the Gym Leaders, you have N to promote a shifting, malleable team. His ever-rotating roster ensures a constant flow and rotation for your team that Green, as an
example, never did.
N was excellent, yes. Usually a much better challenge than any of the nearby gym leaders.
Also, the Unova Dex is far and away more balanced in typing than the Kanto Dex ever was, in so far as it actually has enough Pokémon in any given type for a leader to have a cogent and varied roster.
To an extent. Unova Dex is shit for Fire types, though. Kanto is poor on Dragons and Ghost types, although back then Dragon was the "boss" type given no trainer prior to Lance ever uses a Dragon type and Dratini and Dragonair are extremely well hidden (Super Rod in Safari Zone with a difficult catch rate), so that's an excusable omission. Similarly, Ghost types were related to the Silph Scope.
The problem with actual Pokemon chosen, rather than typing, in the early gens, is not really the Kanto Dex's type variety, which is pretty good considering Kanto has a 150 dex and is somewhat on the smaller side, but more the fact gym leaders just seem to use 5 of the same Pokemon line for no apparent reason. That does bug me.