Workbench without DesignModeller is not worthwhile, in my opinion. DesignModeller is an analysis tool more than a CAD program; it has a lot of great functions which help the analyst clean up your geometry for modeling. For example, there are tools in DesignModeller specifically to help the analyst detect and remove slivers, chop up geometries for meshing, or create shell bodies.
If you're planning on doing your model manipulation in a CAD program outside of Ansys, I'd suggest that Workbench has only limited benefits over the traditional Ansys environment (Ansys APDL). I see the biggest advantage to Workbench as being the unified environment for working on models. Classic Ansys certainly has more functionality when it comes to more complex problems. APDL allows users access nonstandard element outputs, access results for subsets of nodes on the fly, and to parametrically adjust just about anything in your model for optimization purposes.
Of course, all of the extra functionality of the Classical Ansys interface comes at a cost: It has a somewhat steeper learning curve. A veteran analyst will tend to use the Ansys syntax to create useful, elegant, fast-solving models which answer critical design questions more quickly and effectively; however, it takes a while to pick up the user interface and to learn the Ansys Parametric Design Language. That's not to mention the time that it takes to learn different analysis techniques and which technique best represents the case at hand -- which is a software-independent skill.
Regardless of whether you get Ansys with DesignModeller or not, it's great software and I'm sure you'll be happy with it. Good luck.
Christopher K. Hubley