The same advice was given back in the "bad ol' days" of the early, high dosage pills (YES, GIRLS, I am old enough to remember when BCPs
first came on the market!, and took pills with dosages that would make you swoon (not
literally, but because they were so high by today's standards.

)
The original rationale had nothing at all to do with dips in therapeutic/effective blood levels (shoot, we were taking enough hormones for an elephant ...

) -- the idea was purely, simply, to get yourself into a convenient, familiar routine so that you
would be sure of taking the pill every day.