There's no situation where a teenager needs to drive over 80, probably; that only occurs on the highway, and most parents probably aren't going to let their teenagers drive on the interstate.
While I didn't drive on the interstate for my first few months of driving, by the time I got my license, I would drive on the interstate rather regularly: somewhere on the order of 90% of my trips.
The interstate is a great deal faster than the side roads, with less chance of getting lost, as well as being noticeably safer in several ways. You have two basic options for types of roads in many places: The first is about ten to twenty miles per hour faster than the other, and the second has lots of intersections, many of which are unregulated, and is rather twisty at times, as well as being small, and perhaps not as well lit or maintained. Would you take the interstate or the side streets? Which would you want your kids to take?
While the interstates are certainly faster, and while I would never put a brand new driver on them, I would tend to believe that they are otherwise safer for less experienced drivers under many circumstances than an alternate route composed of side streets.