At some point instead of shipping a product to a customer and doing some postconfigurations afterwards one decides to create an installer, which would handle all the process. Of course, having an installer is not an easy task -- installer needs to be changed along with your product. Testing the installer and supporting uninstallation are not trivial tasks as well..
The thing is that
We have just finished upgrading our VMware -- vCenter Server, ESX on our hosts and virtual machines to the forth version. We expected to spend all weekend on this, but it turned out we managed to upgrade in just about 5-6 hours! And that is without any previous serious experience with VMware! Well, yes, we have only two hosts, no cluster and just about 20-25 active virtual machines. But anyway. We had some problems on our way, but generally installation was not that complicated.