> Because GitLab is open source, and the alternatives (GitHub, BitBucket) are not.

That's not a reason to use the hosted gitlab version, that might be a reason to self host gitlab.

Not really - if you are into open source because of the ideology and not just for the price then GitLab makes a lot more sense.

If I find a bug in GitLab I can feasibly submit a patch for it.

Also, if you need to, you can migrate to a self-hosted setup without switching to another platform. There's a built-in tool that makes this easy.