for us its not really the concern - its the extra work for solving simple problems and tasks in our daily business
For example: People making errors while creating views/dashboards which consume a lot of CPU or even crash CMK (not the first time that happend) - and with 300+ users just a matter of time. You cant quickly become the user and check/fix the view/dashboard.
Also you cant just create dashboards as an admin and quickly assign to a user or contactgroup/usergroup.
That extra way of switching to another dummy user costs so much extra time - more or less would create a full time job!
About your concern, you can delete the user and the folder suppsed to be still there. If you add afterwards again a user with the same name, he will get all the views/dashboards/settings back - BUT this can also clearly create another problem - depending your company and naming schemes
cheers