VMware Converter Usage

VMware converter is free from VMware (I think not the enterprise version). In the new released vSphere 4, it is bundled with vSphere 4. The VMware converter automate and simplify physical to virtual machine conversions as well as conversions between virtual machine formats. Here is an chart maybe useful.

I use the VMware converter in the following way:
1. physical to virtual (P2V)
When I want to convert a physical machine to a virtual machine, I need to install the converter on the physical box first, and I also need vCenter support.
2. virtual to virtual (V2V different virtual machine format)
some of my friends working on VMware workstation, but I usually work on ESX. When I need their virtual machines, I will use converter to convert from workstation virtual machine to the ESX virtual machine.
3. virtual to virtual (V2V same format, different setting)
I just do not understand why VMware does not allow me to change the virtual machine hard drive size in ESX after I have created a virtual machine. So I have to use VMware Converter to shrink to hard drive size or expand it.