Yet another third party service to depend on which has unknown reliability or future business model plans.
After spending time with Docker, I like it when it's deserved, but my first reaction is one of "Uh, it's one more layer, it will bring new tooling and required knowledge and error conditions, let's only do it if we have to".
The thing that I like most about terraform is that ultimately, it's just a tool. Just something independent and (mostly) pinnable & controllable that runs on peoples' machines.
> Which, mind you - is optional
Everything starts out as optional before they start ramping up the monetization model.
But of course, it's an open source project, so wherever Hashicorp want to take this, the community can just fork it. However, it's clear where the center of gravity for future development is going to be - towards pushing Hashicorp into your critical path.
[1]: https://www.terraform.io/docs/backends/types/azurerm.html