I think I have another analogy to redneckgurl's likeness of Hermoine and a Rook. A rook is a cornerpiece on a starting board, corners hold a certain amount of authority in literature - the four winds, the four corners of the world, etc. But a corner is a pillar, something to hold everything up, Hermoine and her knowledge helped Harry against almost every trial he has faced. Another reason Ron (JKR) placed Hermoine in that spot instead of the bishop's spot. JKR doesn't do too much by chance.....