If you had thousands of mac minis in stacks you would require a team of technicians which their job would be to go around troubleshoot hardware due to lack of remote management. Things like like plug and unplug things - figure out why a particular mac mini is not turning on in a certain rack, not to mention how do they know that they are looking at the correct mac mini among the thousands? They all look the same and there is no ability for a remote administrator to light up a UID on one.
Another simple but good example of server evolution. A personal computer sitting in a rack does not become a true enterprise class server.
The best kind of server is the one that you can rely on.