The alternator is always on regardless...it's not mechanically regulated. There is always drag regardless of the current pull needed by electricals. There's no clutch system like there is with the a/c. Most a/c systems at WOT application, turn off for a set amount of time, say 15 seconds. Yes it helps power due to parasitic loss, but it also prevents over pressurization of the system (a very baaad thing). The a/c has no effect on alternator output. A/C is completely mechanically driven EXCEPT for the fan logic, which may dictate that fans come on (if electrical) to keep temps at check due to higher loads. I see this as one of many advantages to electrical fans.

On a side note, I used to live in Vietnam and many locals would often drive their Honda Om's (little motorcycle scooters) with the lights off at night believing they'd get better gas mileage. Of course it wasn't true, but you couldn't convince them of that. Subsequently, there are many many deaths. Enough that the government actually issued a statement on it (no taxation so they couldn't really start a campaign)

I know I had a bried conversation with Tone some time ago and he didn't measure much of a difference between electrical and mechanical, if any so he went back to mechanical (if I remember correctly). But I can't ignore the fact that every modern sports car currently has electrical fans. Not only can you more finely control temp with variable fan speeds but you open up a buch of room too.

The only real caveat that I see with the electrical setup is that most modern cars with electrical fans use the ECU to control the fan's functioning, ie there's lots of different inputs and logic behind how they operate. From just one fan at low speed to two fans at high (assuming a two fan system). I know one fan manufacturer (it was probably SPAL) had a new, fairly sophisticated unit on the market but I doubt it would cover all the bases like a properly-designed, OEM from the factory system would.