Kinda disagree with this. I'm of the opinion that 18 agi is a waste unless you want a slightly tougher shield or you want to use a steel shield. WPF itself also scales terribly and isn't a good investment. With 4 WM you get 139 WPF at level 30. With 6 WM you get 156. In actual play the difference is negligible. My polearm guy with 12 agi and a 90 speed weapon doesn't get outspammed. Also, athletics doesn't make much of a difference if you're wearing much armor. There's not much difference between 4 and 6. An agi spammer won't be able to circle around you if you watch your footwork.
There are also many successful players with no agi whatsoever and don't get flanked or outspammed. With the current iteration of this game, str builds outperform agi builds.
Swinging a bit faster isn't just to outspam the enemy, it makes manual blocking harder for them.
I find it rather easy to block a long and slow polearm. Even if I miss my block, I can still correct it in time. But when fighting a much faster weapon I can't do that and I have less time to react. I make more mistakes.
Other than that, I agree with most you've said.
My 2 cents on topic:
24/15 is great, but has distinct disadvantages. Works best in low armour, which makes you vulnerable to getting 2 shot by archers or taking 25% damage from a horse bump. This build is best used at level 31.
21/15 is a very good build. Great damage and enough speed. Gets too slow when you wear heavy armour.
18/18 is a nice allround build. Good speed on foot and good weapon speed. Damage isn't a problem, especially when using blunt/pierce damage or a masterwork sword.