Finishing Lost Chapter

Why do people rush finishing this item's build path? I don't understand. I always save it until like 3rd or 4th item to finish into Ludens. 20% max mana EVERY TIME you LEVEL UP sounds a lot better than a flat 300 mana.

Whenever I play I feel like the sustained spell damage far outweighs the stack damage from Luden's Echo.

Does anyone actually feel like it's better to finish first item LE?