there are a number of reasons to split clusters the link to Duncans blog highlight them from a technical view point, if you are not using Blade technology then I would not bother spliting.
Another major reason is license compliance, as the other poster says, this is usually for a Oracle Cluster.
by spliting up your environment you will be reducing your overall resources as you will need to introduce more failover hosts, currently you have two hosts assigned for failover purposes, if you split into three seperate Clusters then there will be a requriement for an extra host to be assigned as a failover host.
overall this seem a waste of resource.
you say that your environment is looking at larger VM's this is just a capacity planning exercise, I would continue your arguement, as you are on the right lines.