[Si Coe]

You've also got to consider that not all electrics are equal. For example, my 210 runs without an underservo weight, because I'm using a fairly heavy DS1015 servo anyway. Other people using light low profile servos need that weight just to get the same balance as me.
Weight differences between Lipo's especially is quite significant, so you are always going to need some adjustment.

Incidentally many real life racing cars are made underweight and then ballast is added. It lets you tune the weight balance (as mentioned) and you can add and remove ballast until you are exactly on the limit.

In 2wd RC's though its more down to the fact that the balance of a good fast, jumpy dirt track car is very different from that of a tight grippy astro car, so all designs, even convertible ones like the 22 and 210 are naturally a compromise.