Everything is in P4: you checkout the project to work on it, you have everything. You update, you have everything up to date. All the tools are there, so any part of the pipeline can rely on anything that's checked in. You need an older version, you just check that out and off you go. And you have a single repository to maintain.
VCS + Nuget: half the things are in the VCS, you checkout the project and then you have to hunt down a bunch of packages from a separate thing (or five), when you update the repo you have to update the things, hopefully you don't forget any of the ones you use, scripts run on a prayer that you have fetched the right things or they crash, version sync is a crapshoot, hope you're not working on multiple projects at the same time needing different versions of a utility either. Now you need 15 layers of syncing and version management on top of each project to replicate half of what just checking everything into P4 gives you for free.
