Hi Bob, We've never gone this route for a number of reasons (at least yet), so I have no experience in doing this myself, but just to add to this discussion a space can be set permission-wise to allow for anonymous users to access it, which some org's use for external-facing sites. In other words, no logging in is required, therefore no email address. Perhaps this possibility was already dismiissed but wanted to share it with others who may be reading your question.
Like Nic said, they would not receive notifications and such, so there would be a loss of those users being able to use some collaborative features..
Here's an Atlassian blog post from a few years ago explaining some nuances of anonymous access:
Hi my Community friends! For those who don't know me, I'm a product marketer on the Confluence Cloud team - nice to meet you! For those of you who do, you know that I've been all up in your Co...
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