The original "roll 3d6 for stats" that was introduced in 1rst edition used the dice to set up a bell curve that was supposed to represent human (or dwarf, or elf, or whatever) variation. The problem with this is that 3d6 does not have enough range to truly represent all variation within a population.
For example, rolling an 18 for intelligence is supposed to mean your character is a genius. But... 18 on 3d6 is a 1 in 216 chance. This is less than 3 sigma above average. An 18 intelligence human would be right on the border of being able to get into Mensa (which is top 2%). This doesn't even come close to the genius of a John Von Neuman, a person who in the real world did roll his stats naturally...
The addition of d100 to strength mitigated this to some extent, but at the top end an 18/00 strength was still less than 4 sigma above average, and probably not at the level of current day elite athletes.
My purpose for pointing this out is that if we are to assume that player characters are exceptional, then it is not a problem at all that they are outside of the 3d6 bell curve established for racial norms, since that doesn't really have enough range to cover exceptional abilities anyway. This is why the change to stat bonuses does not really bug me. It wasn't a good model of the range of abilities in the first place, so granting more freedom to make the character you want is a worthwhile change.
