Virtual Directories and Persistent Cache

I got drawn into a debate lately about the pros/cons of persistent cache in a virtual directory, and the practical implications of it. (I know this is an old debate. Better late than never?) A persistent cache is basically storing a copy of data locally at the virtual directory, so it doesn't have go get the data each time.

The first question is 'why add this capability? isn't the whole point of a virtual directory provide real-time access to backend data?' In my conversations, I basically received one answer: performance. Virtualizing and transforming the data can slow things down a bit.

Clayton Donley makes a case against persistent cache in an older post. To summarize:

• Persistent cache will mean data isn't real-time, which means the 'freshness' of data will be compromised.
• There are security concerns with adding another place to keep the data.
• There is pain associated with managing yet another directory.

(i.e., if you want a metadirectory, then get a metadirectory!)

So, I've come up with a few questions, and was wondering if anyone has any thoughts about it...

1. Since performance is the main point here, does any one have numbers on the performance hit caused by virtual directories?
2. Is performance the only real justification for persistent cache?