Here is what I think... Inventor is a good tool to use but Autocad is the only software that has been used in the industry (there are some companies who uses inventor). Yes... it is easy for us to draw in inventor but Autocad is a really powerful tool.
Few examples teams using Autocad to design a robot:
I personally use inventor and have drawn bunch of robots. But, I haven't been able to find a Robot cad drawing like this in inventor (like swampthing's and Pink's).
Autocad does have it's own way to weigh the robot. You just have to know what you are doing.
Tytus: I personally would like to challenge you to find a robot drawn in inventor that looks like this.