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,