ok, i see where you are going now.
yes, the hypervisor that is esxi is free to install, but i have never met a company that is doing just that, downloading the free product and nothing else.
ESXi is used by all the versions of esx virtual infrastructure, the one you have has very little management and to have to log into each host separately is just defeating the object of the whole product.
Plus, are you telling me you are running mission critical servers on esxi with no support? That isn't a great idea, i#d say support is mandatory, especially with a product like this where so many applications are relying on a single server, getting it back up again is even more important, especially if you don't have another server to vmotion it or HA to.
You really are being cheap tbh, just using it because it is free, but not using it for any of its great features and not having support and being able to backup the vmdk's.
Also, to get virtualcentre you would have to purchase at least the foundation license to get the agent license and then also the VC license itself and support which is mandatory (as it should be). Being able to see and manage your servers from a single pain of glass is one of the major benefits of virtualisation.
So, technically, yes, you are right, but no-one in their right mind would run mission critical servers how you are doing, madness, and it will come to bite you in the ass at some point.
VMware is an amazing product, but you have to use the amazing features to experience this, just using ESXi and nothing else is just wrong. they give it you free so that you can download it and then upgrade it to a virtual infrastrucure license, and as i've said, i have not heard of a company doing what you are doing.