I think that you make a "fair" point Eli.
However, I would counter with the opinion that effectively extending build season until a teams first competition would do more harm than good. If a team is indeed building their robot in a garage with hacksaws and hammers I would be of the opinion that 1-5 more weeks of build would have little impact on the final robot that competes. It is not their fault, especially if they are rookies, there is just only so much you can do with limited resources.
On the other hand, if you give a team with high end manufacturing capabilities, and a comfortable budget 1-5 more weeks to refine a single robot I think that the end results would be astonishing. The bag day provides all teams the same deadline, and the withholding allowance gives all teams the same opportunity to make changes to their competition robot at competition, or during a work window.