With VMware tools you can do some things easier. For example, you can do "clean shutdown" of VM from ESXi, or synchronise time between ESXi and VM. I'm not sure but I think vmware balooning driver needs tools to work properly.
But you can still live without vmware-tools and do all the above mentioned tasks (althought not so easily). I personaly do not use vmware-tools with linux-vms because modular kernel is not allowed in our company.