This is all the fault of qaz. And long time reader Nat had a blog post on this ages ago.
First, I shouldn't have to remind you all that much about a simple fact of nature in the academic crediting system. Citations matter. Our quality and status as academic scientists will be judged, in small or in large ways, by the citations that our own publications garner.
This is not to say the interpretation of citations is all the same because it most assuredly is not. Citation counting leads to all sorts of distilled measures across your career arc- Highly Cited and the h-index are two examples. Citation counting can be used to judge the quality of your individual paper as well- from the total number of cites, to the sustained citing across the years to the impressive-ness of the journals in which your paper has been cited.
Various stakeholders may disagree over which measure of citation of your work is most critical.
On one thing everyone agrees.
One problem (out of many) with the "Supplementary Materials", that are now very close to required at some journals and heavily encouraged at others, is that they are ignored by the ISI's Web of Science indexing and, so far as I can tell, Google Scholar.
So, by engaging in this perverted system by which journals are themselves competing with each other, you* are robbing your colleagues of their proper due.
Nat observed that you might actually do this intentionally, if you are a jerk.
So now, not only can supplementary info be used as a dumping ground for your inconclusive or crappy data, but you can also stick references to your competitors in there and shaft them their citations.
Try not to be a jerk. Resist this Supplementary Materials nonsense. Science will be the better for it.
*yes, this includes me. I just checked some Supplementary citations that we've published to see if either ISI or Google Scholar indexes them- they do not.