From:
http://www.sgi.com/tech/stl/alloc.html
This is probably the most questionable
design decision. It would have
probably been a bit more useful to
provide a version of reallocate that
either changed the size of the
existing object without copying or
returned NULL. This would have made it
directly useful for objects with copy
constructors. It would also have
avoided unnecessary copying in cases
in which the original object had not
been completely filled in.Unfortunately, this would have
prohibited use of realloc from the C
library. This in turn would have added
complexity to many allocator
implementations, and would have made
interaction with memory-debugging
tools more difficult. Thus we decided
against this alternative.