[IronicDeadBird]

It is all basically about leveling the playing field. Most motors they allow fall into a category and each category has roughly equivalent utility and pricing. This prevents "Super teams" (teams that were bitten by a radio active NASA mentor)* from getting hardware that completely outclasses new teams.
The other points you brought up are absolutely solid, and I hope that the GDC is working or has a group working on refining these building constraints that generally stay consistent year to year... Minus the unlimited motor thing. What I find surprising is that every year the pool of motors should expand to include more options providing more versatility in builds but it doesn't.
Then again the mentality of adding versatility through expanding an options pool is a video game thing not a robot thing...