As a third party service provider you could still host and/or modify those CI runners for specific tasks like node, go, etc.
And to give some additional background to dominotw's question. We didn't like the options for CI on-premises. And by integrating it directly into GitLab it is very easy set up a new project, encouraging people to use CI on more projects.
Having people use GitLab CI will mean GitLab is a less attractive platform for other CI products. We do have a good commit status API since GitLab 8.1 and there is a great Jenkins plugin that supports that http://doc.gitlab.com/ee/integration/jenkins.html