Chef is commonly used for provisioning servers, right?  So is LibCloud, right?
What's an example use case of why someone would use both tools together?
I use chef to bootstrap ec2 instances.  I also use boto to do further modifications of ec2 instances such as creating tags etc.  I will be now be using libcloud more often since I will I will have a mix between rackspace and ec2.
As a side, when bootstrap a ec2 or rackspace instance, I do not use knife, I use libcload to boot a machine and ssh into into machine and install chef client since I fond it to be more reliable and even faster then knife by 3-5 minutes.
Net net, both are used together. Its a happy marriage.
Example of using Chef with LibCloud