I also believe that any young girl who has developed an awareness of high fashion will want to buy the 'real thing' - Vogue/Elle/Bazaar - instead of some teenage stopgap publication, which might not seem adult enough for their aspirations.
On the other hand, free stuff - or 'tat' as I call it - always helps to sell a publication. There are times when I have bought a teen weekly mag for the free gift, and I'm twice as old as the intended audience. But if a 14-year-old yearns to read about Paris couture, free bubblegum lipgloss isn't going to satisfy her.