  Although excitons are not exact bosons, they are commonly treated as such
provided that their composite nature is included in effective scatterings
dressed by exchange. We here \emph{prove} that, \emph{whatever these
scatterings are}, they cannot give both the scattering rates $T_{ij}^{-1}$ and
the exciton lifetime $\tau_0$, correctly: A striking factor 1/2 exists between
$\tau_0^{-1}$ and the sum of $T_{ij}^{-1}$'s, which originates from the
composite nature of excitons, irretrievably lost when they are bosonized. This
result, which appears as very disturbing at first, casts major doubts on
bosonization for problems dealing with \emph{interacting} excitons.
