True that some hard surface shapes are more difficult to model, but you can do a lot of great work in ZBrush alone. Regarding sculpting, painting and rendering, the answer is definitely yes you can use ZBrush alone, or at least with a compositor. I used to just use it with photoshop and it was a ton of fun. While I didn’t need photorealism, I never felt creatively limited.
Once I started using modo, it just became a bit more fun for many reasons. Along with the renderer and ease of making base meshes, I could then incorporate hard surface elements into my work easier.
BUT, keep in mind that you can really sculpt a lot of complex hard surfaces. In fact, some times it’s the better approach! With the lazy mouse and many other features, you can accomplish quite a lot hard surface vs. organic contrast. Here are some examples:
http://www.zbrushcentral.com/showthread.php?t=075105
http://www.zbrushcentral.com/showthread.php?t=070974
So you can sculpt them, but also use the projection master. Here’s a tut (7th one down):
http://www.pixologic.com/zclassroom/homeroom/
Many advantages come with learning a dedicated modeling and rendering package along with ZBrush, but I just wanted to point these out so you don’t feel limited with what you can do in ZBrush. Good luck!