#IPNI is a collaborative project between three august bodies in the taxonomy of plants. They are the Royal Botanic Gardens, Kew, the Harvard University Herbaria, and the Australian National Herbarium.
There are three areas where IPNI sets the standards: plants, authors and publications. The objective is to disambiguate any taxonomic reference to a plant in scientific literature to the correct taxon given the taxon name, its author information, publication information and date.
IPNI publishes several graphs indicating the success of their work. I have been involved in this work as a consequence of a database project I did for my father who loved his cacti and succulents.
One example of what information IPNI provides can be found in this page for the "genus" Echninocactus. In my understanding, the correct full taxonomic name is: "Echinocactus Link & Otto Verh. Vereins Beford. Gartenbaues Konigl. Preuss. Staaten 3: 420. 1827". It has all the required information, it has type information, it has links all as you would expect of a standard like this.
To appreciate the work of IPNI; in stead of "Link & Otto", there may have been: "Link and Otto" or "Link et Otto" or ... obviously the information for the publication is easily made into a different abbreviation.
Wikidata included only a subset of the full taxon information. It is easy enough to understand why; Wikipedia only needed the most current one. It is an easy model; works relatively well and it breaks in the corner cases. With the development of WikiCite there is a great and possibly easy opportunity to expand on the current work given the expanding collaboration with botanical partners like the Biodiversity Heritage Library.