Cube I agree, but i build a lot of very small ships, most are so small that I REALLY can't fit in another 2x2 part without making the design significantly weaker or more expensive. Also part of the reason is that the code required for this (think MassMaxUse=140) could in some occasions be very useful for creating tiers in modding. For example restricting small reactors and ammo factories for use by small ships only.
I have two viewpoints for this. On one hand, I agree with you that using mass as a way to restrict certain parts could be helpful for things like tiers in modding (like you said), but I also have a disagreement with this. For example, take the Fighter weapons from Kroom's Forge. They have more than just a use on a fighter, because you can make makeshift turrets or similar from a combination of the fighter guns and armour. If you take that away and force parts to only be used on a certain weight (or tier) of ship, you are restricting creativity.
Now obviously, that example is rather specific, but while we're talking about fighters, let me address this another way. Sometime in the future, customisable fighters will be added to the game and from the looks of things, the logistics behind them are going to be different than the terran ships we're used to. For starters, they probably won't have crew running around in them supplying the weapons, thrusters etc by physically taking a battery and plonking it in the part. There's a high probability that fighters will have automatic distribution, which includes storage of power, ammo etc. This will mean that they'll have their own parts.
When I say this, I'm thinking about future proofing. Why would you make parts that are designed for lower tier, lower size ships, when in the future, a different system for the same (or similar) type of ship will be in use? Is it worth the trouble to implement such a feature? In terms of superweapons, perhaps a mass threshold where you must have X amount of mass in order to place the superweapon, it would be useful, but for fighter parts that might not necessarily be needed in the future, I'm not sure.