On our old server, we had multiple instances of users running benign yet out-of-control processes that ended up allocating all available memory. Shared daemons, like our web and mail servers, would then crash when trying to allocate memory, creating denial-of-service for everyone. The ssh daemon wasn't even able to work properly without available memory, so we sometimes needed to ask the techs at our hosting facility to reboot the server for us! It's always better safe than sorry when it comes to protecting users from breaking other users' services.