The argument for PC style virtualization, in which a hypervisor switches between OS/application bundles on demand, is extremely simple: license something like VMware ESX to run multiple copies of the OS and therefore strongly separated applications on a single machine, and you can use the same server for multiple jobs, thus achieving hardware, power, and space savings.
Great, except that:
The most important objection, however, is a practical one: there's a much better way - actually, two of them.
In both cases the better way is simply to allow the OS to do its job - and if yours isn't up to the job; change to Solaris, because it is.
Part of the issue here is that the popularity of PC style virtualization responds to issues the Unix community has never had - the pretend professionalism implicit in copying data processing's commitment to partitioning, and the nineties NT manager's learned aversion to trusting NT with more than one application at a time.
Notice that both of these illustrate what happens when people refuse to adapt as reality changes - the cost and memory management issues that drove partitioning and VM in the 1960s were history by the late seventies, and today's Windows servers can easily handle a number of concurrent applications provided that the load process leaves the registery in a consistent state and no more than one hacks it during operations.
Solaris handles the job in several ways. The most effective is to simply load your applications and run them - just use the parameter passing functionality built into start-up script processing to ensure that you don't have port, storage, or shared memory conflicts.
The alternative is to use Solaris containers - basically virtualised environments that share the same kernel.
Either way, you get concurrent operation for multiple applications - but only one OS, and so you get the space, power, and hardware savings promised by virtualization but without the off-setting administrative cost increases and without necessarily giving up instantaneous user access to the full resources available.