[plt-scheme] Question chapter 10 box issue

Emre,

PLAI uses a box for several reasons.  One useful reason is because, in
Part VI, we do mutable structures (a la boxes) before we do mutable
variables.  There is a reason for that ordering in the book, which you
can divine once you have read those sections.  Anyway, given that,
from the book's perspective mutable boxes are "simpler" than mutable
variables, so that's one good reason to use boxes.

Shriram