I would argue that it doesn't even do much to pad a resume.
The career slam is merely the product of recent media hype--struggling to conjure up reasons for American fans to return to watching tennis.
the career slam was worth something when agassi did it, no doubt
and of course, because agassi played hard court tennis on clay and was not particularly good on the surface (those early FO finals and that Rome win are misleading)
he relied on his magnificent hand to eye co-ordination for those returns to win wimbledon 92, he wasn't a natural grass court player either
but it was worth something, purely because no one had done it in 30(?) years
edberg and courier came close
edberg with 2 AO, 2 WIM and 2 US, FO final
courier with 2 AO, 2 FO, WIM final, US final
but had edberg won it, would he then be above mcenroe, both having 7 slams?
however, in the context of a career overall, it means less
then of course, you have federer and nadal (and nearly djokovic) achieving it in the space of a few years
this is due to slowed down surfaces (or sped up in the case of clay), the same prevalent game style and the same players winning every tournament that matters
it still means something to win at every slam, but (nowadays) overall slam counts, weeks at no 1, and total titles won, those are the main deciders in tennis greatness