roles change, updates happen frequently, and I'd rather a machine spin up with the latest packages.  I've always found that updating a pre-built machine is slower, sometimes substantially so, than just building a fresh image with the newest rpms.

That said, some roles can (and often should) be fairly rigid and slow to be updated.  But there's not much less of a need for flexible, dynamic builds in the cloud than there is in a local server room; do you build all new local servers based on a pre-built image that you just replicate?  Would seem to negate the purpose of a kickstart server ;)

Brian