They are topical to focus interest and to let the author develop a linear story of progress. In all these ways, they are responsive to the new objective. And in these ways I think they succeed. Nonetheless we should not forget that the primary readership for these reports has been and always will be the SPA community, whose interest is the traditional one served by the former reports. This interest has been somewhat compromised.
Topical reports mean that areas get missed.
Also topical reports are not sure sources for references. These compromises are the cost of the experiment, yet no one denies that the experiment is worth doing. Next quadrennium's collection of IUGG reports could read like an SPA issue of Scientific American, and that wouldn't be bad.