Can someone explain...

why database caching is non-existant when using a different database per request as described in the following how to?

Assuming I have one database per customer, I can see how the application may not be able to cache. However, the databases themselves will continue to cache and, in fact, the MySQL query cache will produce better results as an update to a customer will only invalidate the query cache in the database for that customer.

I appreciate any additional insights, Paul