No server is required to use them, and the amount of spam and fraud they filter out is enormous.
Okay you do have a point. The thing is they get abused for email where it's pretty much a racket. I just really hope Lemmy doesn't end up the same way, since if some bad faith powerful actor starts having control over a list then they get to dictate which servers can federate and which ones not, which is pretty much a walled garden.
I do get the need to identify malicious instances preemptively though, spambots are a threat wherever we go and some instances are just insufferable like exploding heads.
My respects to you. I really think websites and software in general need to have efficiency in mind again. Webapps should use as few resources as possible, save people's time, their money and the planet.