Yes, good point.  
Auth + Rate limit = Great. They might be good enough for quite some stuff.
But "Worthiness of a free trial" translates into "Likelyhood of using it paid" real fast when you look at products/services that are social, like many Nostr things inherently are. 

Letting tons of people try out your Tennis club for free might actually be a great idea on a specific "Discount Day" or something like this. But you don't ask your paying club members to invest time and energy into a bunch of new people aaaaaall the time. I've lived that life for a while in an ecovillage and it's bad UX.
My guess is that, just like for those real-life examples, you don't simply let people in without some sort of skin in the game.