This is also the most compelling aspect of it to me as well. Domains being essentially free allows for too much easy spamming of internet users. Raising the cost of an identity online really helps solve a lot of the bad actor problems.
My experience with spam [1] is that 90% of the non-trivially-filtered-kind comes from either taken over 'mainstream' email provider accounts (eg. gmail, yahoo, ...) or from exploited websites that can send email (eg. a wordpress or prestashop instance). Obviously dedicated 'spam' domains tend to get blocked _very_ fast on a multitutde of RBLs. It's the domains shared between spam and non-spam users that are the real problem.
Making domains more expensive probably won't help fighting spam. It will, however, increase the barrier to having an online identity to people who can't blow a couple months' rent on an urbit identity.
On the other hand, being able to register domain names without _any_ identity checks and oversight from legally responsible registrar entities could possibly increase abuse in some other ways.
[1] - I help run a mail server for a hackerspace.