Here's our schedule for the last two seasons:
Weeks 1 and 2:
Hardcore design and CAD work, resulting in a full, functioning robot model and part prints. Mock up kit chassis for programmers to start tinkering.
Weeks 3 and 4:
Fabrication, assembly and testing of practice robot. Noting design changes as we go.
Weeks 5 and 6:
Fabrication and assembly of competition robot.
The definite (and as I feel most important) advantage to building a practice bot is drive practice. Confident drivers make less errors which results in better performance on the field.
You could also say that it gives programmers more time, but you could have a 12-week build season and the programmers would still want more time.

(sorry Tubby)