I suspect spam on global made it difficult to just passively lurk. Lurking is a big part of the social media experience. You don't want to speak until you know who you're speaking to.
If Iris didn't have a good client side way to filter spam, and participate in any/all relays openly, but with a decent global, I wouldn't have gotten as hooked. Other clients seem to focus on Followers only as the experience, or paid relays as a solution, and those fail the user, in delivering a social experience.