I got this back from one of the developers. It really is a legacy/old convention. We have update the BIOS 6.x .h files to do the proper include so that this isn't necessary. For DSP/BIOS 5.x, you need to include <std.h> before any of the other .h files.
Why not always include std.h in every module header? Mostly for historical reasons: since *all* modules use the base types in std.h, it was considered (at one time) too inefficient for this header to be included by all headers. As time went on the efficiency argument held less weight but we now have a number of legacy headers that do not include std.h. Thus, clients can not assume that it will be included by a module header and we must continue to make this *one and only* exception.