Myself personally, I would go with it. Variety is nice, yes, but I feel it’s more important to cultivate a love of reading. And let’s face it – once we’re adults, we tend to also have preferred genres and that’s what we stick with. Also, we learn more from what we are interested in, than what we aren’t.
So, I would go with it, BUT… I would supply the options. There are some wonderful classic adventure novels – Tom Sawyer, Huckleberry Fin, Swiss Family Robinson, Lord of the Flies (depending on their age), Gulliver’s Travels etc…