This is a great idea, both for us as a HubSpot Diamond Partner and for clients that have requested this functionality before.
In our case, we would like to track information regarding our team members and their workload within HubSpot, these custom user properties would be great for that.
I can also remember clients in the past having similar needs too, such as tracking employee birthdays and addresses for internal use.
I am happy to provide any additional feedback that would be of assistance here, and we would certianly be interested in any betas that come out of this development.