The weird thing is that he probably never had to drug women for sex; he's Bill Freaking Cosby! He could have gotten all kinds of women to jump in bed with him for free.
The fact that he would drug them into unconsciousness is the strangest part of all. Maybe he thought that if they didn't know what he was doing, they would be able to tell anyone about it. He probably was afraid of being extorted or splashed in the tabloids. So, he knocks the women out, so they can't really say what happened. That's a pretty demented way to have sex.
He wasn’t doing it to not get caught to my understanding, it was a kink. Supposedly at least some of the women were his actual mistresses he would have sex with normally, but then he would eventually slip them drugs unknowingly. Maybe the drug thing started with trying not to get caught and he found he liked it.
Freegards
Or, he was so inept at performing, he didn't want the women to spread the word he sucked in bed.