gradivus
I believe what Malevolence is referring to is the fact that under the regular modification rules, drones would only gain (Body) mod points in each modification category (power train, protection, weapons, electromagnetic, and cosmetic). Sensor costs (Rating) mod points, so if you rip out a Rating 3 sensor array on a Body 3 drone, do you now have 3 Mod Points (the Body of the drone in the electromagnetic category), or do you gain another 3 by ripping out the old sensor for a total of Body 3 + 3 = 6.
Not being able to downgrade attributes for more mod points means drones are extremely limited in terms of modifications, because the normal vehicle modification rules have much higher mod point costs for upgrading most things. So a Body 3 drone gains 3 points in each category, where under the optional rules it automatically has 1 free point in each category + Body. This may not seem unbalanced until you realize that upgrading Acceleration under the drone rules only cost 1 point and can go as high as base attribute x 2, but under the vehicle rules it costs 4 and 8 mod slots and can only be upgraded by 1 or 2 points respectively.
In other words, using the vehicle modification rules for drones is almost completely useless unless you give them Body x 3 points or some such, which means riggers in Missions are most likely screwed as Catalyst typically don't allow Optional rules.