I don't know about anyone else but riding two up in the twisties is a good work out skill wise on the moto. You've gotta be more smooth on the throttle, braking becomes more challenging, the extra weight becomes a factor in turns and choosing lines, one thing for sure is that if you don't know how to relax you'll be dead tired in no time.
As for a mere mortal like myself, I really don't like to have a passenger when I'm riding on the more technical roads. I think the feeling is mutual since the bikes I own don't have much consideration for passengers...
Two Big Jugs or Four Small Cups. Life's Good.
`08 CBR1000RR "Ayane" - the Shinkansen
`07 S4Rs "Persephone" - urban Monster
`00 SV650 "Ayame" - trackday special
Eh, I use a totally different riding style with passengers and ride much tamer. So while it's still fun, it's not nearly as good a workout or a challenge to my skills. Tried stepping it slowly once with a long time passenger. It was a foot down in a corner that reverted me to my old ways.