I'm in favour of incorporating other traits, really.
It's just that it's much easier to train BD, because below level 250, all you have to worry about is which of the 10 codestones, or how many dubloons, you need. With books, you need to know which books you've read and go source new ones out, rarer ones of which may get pricier and pricier as your list expands. Thing is, once you reach the top you tend to have this monopoly. Someone beats you out, you beat them back. At least, I'd imagine something like that would occur. Does it?
From what I see (with reference to this other thread on the General Chat board, talking about frozen accounts,), I think TNT would try to appeal to as large an audience as possible. Fewer people will concentrate on reading books. If I were a person who trained BD extensively, but not intelligence, I wouldn't be very motivated to work that hard, and thus lose interest in the plot. TNT, being the way it is, would likely want to hold on to as many people as possible.
But I do agree intelligence and book scores really deserve greater mention and recognition. BDers get to shine in these wars, smart pets should be given a chance, too. I just think it won't be likely that such an opportunity will be presented soon. Maybe if we yelled louder?