VisualEditor is not enabled by default on the project namespaces because they are often discussion pages (e.g. like https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)), and we think that VisualEditor currently is a very poor tool for posting comments in discussions (e.g. there is no way to indent your comment). But if on your wiki they are mostly documentation pages (e.g. like https://en.wikipedia.org/wiki/Wikipedia:About), then it's a good idea to enable it.