Effective reuse is key to a successful PaaS solution. There are two areas of reuse worth discussing:
PaaS Instance Template Re-use
While automated provisioning is a key aspect of making PaaS cost effective, those processes can be greatly simplified by basing the PaaS instances themselves on reusable templates. The idea of templates is common across WebLogic and Oracle Virtual Machine and quite simple. Once you have an established instance of WebLogic or OVM running somewhere, it is easy to capture the configuration of those instances as reusable templates. A template can then be used to "stamp out" new instances that are identical to the original. This makes the provisioning process much faster and less error prone.
It is important to also understand that while templates provide reusable "appliances" that can be used again and again, these new instances must be easily modified to accomodate the new instance environment. In the case of WebLogic this means things like JDBC connections, user identities, JMS configurations, etc. To this end, using templates means that the provisioning process can mostly focus on just the differences between instances and not the full details of the instances themselves.
This includes the ability to reclaim hardware and physical servers that may no longer be hosting PaaS instances. When a PaaS instance is scaled down according to lack of usage, it may be readily apparent that the hosted applications may be decommissioned. Compare this to traditional IT practices whereby once a physical server is dedicated to an application, it is exceedingly rare that the server is ever returned to the pool of available resources. This is typically because of a lack of understanding of what the business impact would be if the application were switched off. PaaS provides a model whereby application usage is directly tied to resource consumption and as it increases or decreases, so does the allocation of physical resources accordingly.