Activities need to be always linked to a record otherwise it is not at all useful. Sometimes you want to link it to opportunity but also customer. Sometimes you want to link it to some custom entity but also to customer. I would like to see the linkage to customer be automatic and system populated in the same way there is an Owning Business Unit for each record.
Phone Call, Appointment and email could be linked to customer via Sender and recipient (the first recipient). For task there could be a system customer field that is manually updated.
This way the Regarding field could be used for something else. The customer is always important for reporting purposes and therefore the regarding field is in many deployments reserved for the customer. It would be useful to be able to link activities for something else besides the customer. Especially users in app for Outlook could link the activity to some other record and the customer linkage would be automatic. The user wouldn't have to leave the Outlook to manage this.
In some projects I have built complex workflows to get this done and they don't work very well with integrations. They are usually not very reliable in reporting either.