It depends on what quantity of AC we are talking about to say whether anything is "lost". Power = (current) x (voltage drop) at any instant (whether AC or DC). So when a diode stops current flow, there is no power delivered, used, or wasted. Anyway, the alternator is a more clever design with multiple generating coils and 6 diodes, so current is always flowing out thru one of the diodes. The alternator (tries) to generate 3-phase AC power and the diodes force it to flow out on a single lead as a fairly steady DC current.
What many people don't know (I know TrailBeast does) is that the alternator output is always connected to the battery (key doesn't turn it off). You totally rely on the 3 output diodes in the alternator blocking reverse current flow while the car is off. If they leak too much, your battery can slowly discharge. Of course they always leak to some extent, as do all semi-conductors, so might drain down the battery after a year.
