In a past web forum I did infrastructure for, without sufficient moderation and constant updates to captchas/challenge questions, we would get over 25k signups a day (100 legitimate signups a day), and over 500k spam posts a day (<5k legit). You'd be sending every single client multiple-digit terabytes of literal spam, much of it illegal spam (CC theft dump sales, illegal online pharmacy, are common spam) and making them dig through it.
Imagine viewing a single thread, and downloading about 5GB a page of gzipped viagra ads covered in the entire text content of a Harry Potter book in a desperate attempt to get past some filter, and then having to filter through to get the one or two posts.