Ahh yes. The vaunted efficiency argument....
I'm not sure I agree about the vendors preferring the days of separate servers, because the rackmounted server market became very cut-throat, and the vendors were not making much money per server, even it they were selling a lot of them.
What virtualization has allowed is vendors to tell customers that they are justified in replacing perfectly serviceable datacenter servers with years of life with brand new, high margin, expensive servers. For the vendors, high margin small volume is preferable to low margin high volume. That's why IBM's mainframe business is still very profitable.
I'm sure that the vendors can produce spreadsheets and charts to prove that they will save money on power, space, infrastructure and support costs by doing this, but that is what marketing people do. It will be interesting looking back in a few years time, but I'm not sure whether anybody will be publishing figures to see whether the savings were realised.
I was working on introducing virtualized systems six years ago in the UNIX space, and whenever we tried suggesting combining workloads so that the average usage of the workloads approached 90%+, we always got tripped up by the customers (separate departments in a large UK bank buying computer services from a central IT department) wondering loudly what happened to their workload if unscheduled peaks in separate workloads coincided. They never liked the fact that in these situations they might get less predictable batch timings than if they paid for guaranteed capacity. The result was that we put hard limits on each of the LPARs, effectively the same as them having systems of fixed size. They could not afford missing critical deadlines by uncertainties regarding job run times.
I admit that this was before it became easy to shuffle partitions live between different physical systems, but it became clear that end customers were not prepared to compromise in order to make more efficient use of the installed capacity.
I'm not involved in such work at the moment, so maybe 'education' or 'marketing presentations' are better at convincing customers nowadays.