Kongming is a great card for white weeinie
Honor the pure is a great card for mono white weenie
Do you really need the anthem to also demand you're not playing another color? I find them niche enough as it is
I wouldn't call them narrow. Like, 90% of the white decks in my meta are creature decks and while not a super high priority for all, these decks would all like to pick up and play them. Aggro and tokens even love them.
And to the other point, I agree with you, that narrowness is more often a downside than an upside, and we should all aim for a cube with most card being really flexible tools. But there is a 'too much'. Imagine the abstract example of a cube where every card goes into every deck of or's color, like all lightning bolts. That would lead to a pretty uninteresting draft environment.
Narrowness has upsides. You are right, why would you make people draft mono white? There are dozend of great arguments to go into boros gowide aggro or into the orzhov lifegain aggro or even draft selesnya tokens/aggro or an azorius tempo deck maybe. So a card like Glorious Anthem would not pull you into mono white in most drafts, because you just saw that
Skyknight Vanguard or something. How often in a draft does it happen, that you just feel pulled into one color? If you want monocolor decks be a regular thing, I think you should make them tempting in some way.
For example I support black devotion as an archetype with 6 cards that get better in mono black, two of them even unplayable in anything with less than 16 swamps. People would just not not go into mono black, limiting the pool of cards they can pick and play during the draft, if it wasn't for tempting payoffs like
Crypt Ghast. Just like people would not go out of their way and draft a deck with 16 instants/sorceries if there wasn't that
Rise from the Tides. Without narrow cards people would just draft good stuff decks.
So a card like Benalish Marshall now can lead people to draft a deck they wouldn't otherwise - if it is tempting enough in the meta. Momo white plays and feels different to boros or orzhov, so it's cool to have that additional variance. That was my thought behind that fight club - without creating a larger archetype around this, like I have in black. Of course you can't have too many cards like that, since they will make too many dead cards in packs quickly. But a few narrower build around cards, that can by their own make people draft a little different, make a cube meta better in my opinion.
I've made the same positive experience with
Vernal Bloom as my only card really encouraging going mono green.