Fully agree with the previous comments.
Some arguments, on top, I like to use for myself:
- PowerCLI is administration of the vSphere environment, while vCenter is production, don't mix those 2 as a best practice
- PowerCLI and PowerShell have some prerequisites that do not necessarily run by default on a vCenter.
- There is always the possibility that you at one point decide to run vCenter as an appliance, then you will need to find another box to run those administration scripts in any case
- Less applications on a box = less risk. Think appliance !
- There are more backoffice applications that can be managed with PowerShell (Exchange, SQL Server, Lync...). Keep the administration functions which are PowerShell based together. Makes it for example a lot easier to manage your scheduled administration tasks.
- Some PowerCLI functionality runs (better) in a 32-bit environment, which is not true for your vCenter.
- There are on average 2 PowerCLI builds per year, you don't want to install stuff too frequently on your vCenter server
- Makes it easier to test new functionality, for example while your vCenter might still run on an OS that has PowerShell v2, you can test your administration scripts on a box that has PowerShell v3.