Some Tuleap users are using Tuleap for agile planification and issue tracking only. Right. That's one of the power of Tuleap: you can use only the tools you need for each project.
But if you already use some Tuleap tools and are a Git guy running it on an external server, you should consider better use the Git tool provided into Tuleap. It is hosted on the same server. You could be even more productive.