Virtualisation, like everything, has pros and cons
First off, let's be honest here. ISYS's comment "Secondly, you don't virtualise services that require high end performance so sharing resources with all the other clients on a host is not an issue." doesn't tally with what the virtualisation providers advertise. For example, from VMWare's site:
"Run Business Critical Applications with Confidence
Deliver better application performance and availability with less complexity at a lower cost.
Scalability and Performance
With 4x more powerful VM’s, vSphere supports the most resource-intensive applications."
Of course VMWare wants everything you have to run virtualised hardware, that's VMWare's job. Additionally, sharing resources with all the other clients on a host is always an issue. For a start that's why you're supposed to stuff your new servers full of RAM.
I do see that there are pros to virtualisation and certainly in the "longer term" it makes increasing amounts of sense but right now the con does seem to look a lot like "throw out your stuff and buy new stuff". If your current stuff works, great. All the benefits of reduced power use and server space rental *need* to pay for themselves between licence renewals because there's no "it's better now" advantage in virtualisation. It's not supposed to make your server quicker. Your users aren't getting more done because you bought a few new servers and an often surprising number of software licences and that's a massive failing in a technology upgrade.
Spending money solely for the purpose of saving money needs to be done very, very carefully.
You should never forget that all of these trends (why hasn't your business virtualised / switched to Mac / upgraded from XP / let us install and support Linux / gone paperless etc etc) are advertised precisely because someone intends to make money out of you. The companies selling virtualisation software, as companies, don't really care if virtualisation is right for your situation.