I actually agree with this. I'm a fan of Doc Love's stuff (been reading it since 2003) and many of his principles I have found to be dead on.
In answer to your question: Friday and Saturday dates just scream "we are already a couple," which is not what you want to do with a new girl if she's not there yet... which, for those first few dates, she's not. She may not know it, but she actually needs time limits set on those first few dates so she can build up the desire to want to see you more. With weekdays, people have to go to work the next day so it allows the guy to not only have a built-in time limit, but also a workable excuse he can use to shorten the date ("oh man, I have to get home, I got work tomorrow and so do you"). However, with Friday and Saturday it makes it too easy for the guy to try and keep her out late (after all, no work the next day for most), which causes him to prolong the date and risk the chance of making some kind of mistake during those extra hours that will turn her off.
Another great reason to avoid Fridays and Saturdays is because they can be used as an indicator of her building interest. I myself found that if I took a girl out 3 to 5 times, and each one was a weekday date, SHE would be the one to suggest us doing an activity on a Friday or Saturday night. Which, in woman's language (or "w0manese" as Doc Love would say) translates to "I want to go out with you on one of those two days because I see myself as being with you in a couple-type situation at some point."
It's all about patience, man. You gotta leave certain things open in the relationship as a way for HER to send YOU signals that her interest is building, and this is one of those ways. (Note: If you end up meeting the rare girl who works late nights during the week - like a nurse or a teacher who has to prep for lessons the next day - and she ONLY has those two days available, you would take her out then... but still keep those first few dates short.) Hope this helps!