Well, the advantages and this is pure speculation is:
** Less XP advancement, except the Wizard?
** Only One attribute needed to get class. But with, well... I use 3d6 PaR (Placed as Rolled), but other folks might use 4d6RL (remove lowest) and arrange attributes. So, that takes the fun from that.
** No Alignment restrictions: Except Thief, but who cares about them, right? 
** Now that I look at it, the base classes just there for convenience. If you play traditionally, it be 3d6 PaR and if your attributes or low-average, but you still have a Str 9. You can opt for Fighter and still play. But this days some (most?) GM allow for more variety (The varied Methods, noted from AD&D 1E, which caused me serious mental problems, transitioning from basic D&D blue book). I play this game with 3d6 PaR because the rules (especially for Character creation) are only restrictive while using Method I, and based on the old OD&D ruleset.
Granted, this is just a theory and it best get a more official opinion of the question.
