Nophead's excellent Mendel90 is modelled entirely in OpenSCAD. The benefit of doing this is that if we want to scale the model then all we have to provide it are the dimensions of a few basic components - the heated bed, the motors being used, the diameters of the steel rods, etc. The ENTIRE model is then redrawn with the correct dimensions, holes, etc, plus it produces a bill of materials to boot. That's a pretty powerful feature for anybody wanting to produce scalable parametric designs for actual objects without having to start over each time a small component changes.
If it appears slow and clumsy then you're probably using it to do something for which a better tool already exists.
Right, so now we're talking about the same features, one which is as you say provided by a free tool and one that costs real money. Given that this is the RepRap forum and not the "I spent $10,000 on a commercial printer because I could" forum you can probably understand why we like the free stuff...
Put another way, why would I spend all that money on a commercial tool to do the same job that a free tool does perfectly adequately? OpenSCAD is a tool, and at the moment it meets my purposes. I didn't have to fork out any money for it and I can readily reuse my existing skills in writing C style code. When the day comes where I'm struggling with the limitations of OpenSCAD then I'll go find another tool. The benefits of that tool will have to outweigh the learning curve I would need to overcome to use it effectively.
Incidentally I already use Sketchup for other things but it was the limitations I found in that which sent me looking for another tool - which is when I found OpenSCAD. Sketchup still has its advantages in many situations, but I prefer to use OpenSCAD just because it matches the way my mind works - in the same way I'm guessing that a graphical interface like Solidworks matches the way your mind works.
I'm new at 3d modelling (and in this forum, hello all BTW), and when I've decided that I wanted to 3d model the boxes of some of my eletronic projects I've started searching a tool.
I'm mostry an open source entusiast so I needed something that runs on linux (I do own many license of commercial products that run on linux, but I'm not going to use windows at my home pc, sorry I've enough of windows at work).
I've tried many 3d modeling tools like freecad and blender, but in the end (mostly like Replace) I'm better at tinking and coding than at interactive 3d modelling.
I don't know if there is something that openscad canot do that others software can (or viceversa) and for now I don't care, I just know that I was able to effectively use it about 15 minutes after installing it, so it won my HD!