What you are talking about is pulling in a Motion PROJECT which you'll always have to render. If you export a movie from Motion then you won't have to render (provided the properties of the Motion movie match those in your sequence).
The difference is that if you use a Motion project you can reenter the project in Motion, make changes and then those changes will be reflected in your FCP project without you having to re-import and edit. You will have to re-render however.
An exported movie from Motion is just like any other video clip, so if you had to make changes, you'd have to export out another movie and reimport and edit it into the timeline.
Bottom line, if you are finished with Motion, export a movie. If you might make changes, use the project.
Andy