The user registration record can be easily extended by 3rd party addons, and often contains useful information such as suggesting the users signed up with a VPN or proxy service. We don't want to have to review 10-30 successful registrations a day, so will often only notice after they run into issues with the forum rules which can be weeks to months after the initial registration.
At the same time, we don't really care about rejected user registrations nearly as much as being able to check successful registrations after the fact.
Actually the log is great for also looking how much "went into the moderation queue". You could build up statistics of directly published content and content that needed moderation before it was published. Content that was created but never published at all because of spam moderation (either disapproved by manual action or it was directly rejected as it is with the conversations).
You can also find out which users permanently trigger the spam filter that you would never find out either in public areas like forums or user profile messages or private areas like conversations where a user can trigger indefinitely spam keywords which he then changes to get his message through.