about attributes exist to tie together groups of sibling nodes ("about groups"). If a node has an about attribute that is unique (i.e. there is no other node with the same about attribute value, so the size of the about group is one), then by definition that about attribute is useless (for grouping; @GWicke is investigating if there are consequences for the RDFa graph).
At http://parsoid-lb.eqiad.wikimedia.org/enwiki/Barack_Obama?oldid=644858117 there are 1298 about groups in the body. Of those, 803 are useless (have size 1). They break down into the following categories:
- 37 transclusions that don't end up outputting multiple siblings: <div class="hatnote" about="#mwt1" typeof="mw:Transclusion">
- 395 references: <span class="reference" about="#mwt25" typeof="mw:Extension/ref">
- 1 reference list: <ol class="references" typeof="mw:Extension/references" about="#mwt1307">
- 370 list items in the reference list: <li about="#cite_note-94" id="cite_note-94"> (note that these are not mwt abouts, instead they're duplicates of the id)