How is this different from a script with apt-get or yum? I understand there are some, but not that significant benefit. Did I miss something?
Once you start having to download and build source for apps/libraries that aren't pre-packaged you can run into issues about version compatibilities, where files are located, do you need to download additional packages (*-dev), etc. Having a service that takes care of this for you solves a lot of headaches (part of why rayburst got built in the first place).
In my opinion, you either use PaaS or use chef/puppet, depending on your case.
PaaS works because the provider has a standard and forces you to follow them. I don't mean that in any negative sense. I was in SCM for a number of years and know that kind of enforcement is necessary if you want to have any kind of repeatable, automatable and supportable deployments.