According to Taginfo:
`genus=*` has 1,486,071 uses, vs `taxon:genus=*` with 6,446.
`species=*` has 1,948,089 uses, vs `taxon:species=*` with 2,462.
`family=*` has 3,666 uses, vs `taxon:family=*` with 55,815 total; minus the 54,925 `taxon:family=Arecaceae` which were almost all added in just two changesets, leaves just 890.

In other words - `taxon:*=*` is much like `contact:*=*` in that its usage is orders of magnitude less common than the non-prefixed, entirely synonymous, much more popular alternatives.