This. I've started an army of CoS after the release of the battletome. I've never run Empire, Dwarfs or Elves before, but I now like the concept of normal people fighting against big monster, demi-god and supernatural powers.
Regarding the OP, I completely agree, this faction is designed as melting pot... and yet my choice FOR NOW is to go full humans – However, I'll convert some units in order to make them human (for example my Cogsmith is human) and not lose effectiveness.
The reason to exclude some races for my CoS list is purely "aesthetic": I like strong identity in an army and therefore visual uniformity on the table. Someone might argue that colour scheme can fix this, but elves or stormcasts alongside humans...it just doesn't sound visually good to me.
So, this is only a matter of taste... I don't play ultra-competitive games, but CoS give me (and probably others gamers) the opportunity to converting unique stuff (what I enjoy most) and deploy a visually rewarding army. Also, I am aware I'm not using the book in the appropriate way