Some linguists prefer the use of Netherlandic because that term is less confusing for them : language of the Netherlands in the original sense of the word, i.e. the Low Countries by the Sea. Dutch on the other hand simply means: language of the people, and originally also included German.
Written "Netherlandic" or "Netherlandish" evolved from the southern dialects spoken in the culturally advanced Flanders and Brabant of the 15th century. Spoken "N." grew out of the vernacular of the province of Holland, which became dominant after the 16th century. The existence of both a nationality-term Dutch and a linguistic term Dutch tend to create confusion. The terms Netherlandic/ Netherlandish is then seen as covering both 1. the speakers of "Flemish" variants of Dutch in the Flemish part of Belgium, which belonged to the ex- Southern Netherlands 2. the Dutch speakers in the present "Netherlands" ( the ex- Northern Netherlands)
Example: C.B. van Haeringen, a Dutch academic published a book on " Netherlandic Language Research", showing that he included the Flemish and Brabantian variants. See
As to the history of the term Dutch see the OED.
Dutch = Hollandish, or in a wider sense Netherlandish, and even German. The OED's article on "Dutch" can be found at http://www.englishforums.com/English/Dutch/cbprw/Post.htm