Advantage of using AC generators in cars instead of DC generators
No, it's not for efficiency reasons.

DC generators typically have commutators, i.e. contacts with brushes that reverse the polarity of the voltage at the generator clamps every half rotation. In essence, DC generators are just AC generators that have a "mechanical" rectifier.

You can build generators without any electrical contacts between moving parts, but you cannot build commutators without those.

Since such contacts are very likely to fail under constant use, in dirty and vibrating environments, it's very desirable not to use them in cars. I'd also go as far as to say that unless you build a very expensive one, the contact resistance might be higher than what you lose over a bridge rectifier.