How do you mean he was keen on it? All I can find about it, that it was used in one work, the Lay of Leithian. In my opinion (with the limited things I read about it, I admit) it has been a year long proces/evolution in which the 'bad guy' in his different books have been evolved along the story, and in the end Sauron and Thu the Necromancer were all the 'same'.
It almost feels like retconning considering Sauron could not yet take physical form, yet the Necromancer probably will have just that
He kept the name in The Lay in Leithian up until his proposal to have it published by Allen & Unwin, who asked for more material after The Hobbit. They said no and also continually dismissed The Silmarillion during his lifetime. He also mentioned the name in rough plot points while drafting the 1960 Hobbit. Why continue to use it if he wasn't keen on using that name for The Necromancer.
This is one of many topics that was discussed at the Tolkien Society's Return of the Ring event in August by fellow Tolkien scholars far beyond me in expertise and knowledge of Tolkien.
It's actually one of the most controversial and most debated issues in scholarly circles. Whether or not later musings/theories/writings by Tolkien have more weight to them than older writings by Tolkien already published. The general consensus is that latter writings do indeed supercede older writings. For example; Tolkien's theory as to the origins of the Orcs. In the published material, it's supposed that the Orcs were Elves taken in by Melkor before they encountered Oromë and hence made the journey to Valinor, but Tolkien's latter theory supposed that they were originally degenerate men taken in by Melkor when he appeared to them as a saviour in fair form and in the process corrupting the Gift of Iluvatar (death, freedom for the soul, not to be bound to Arda).The dark history that the Race of Men are so reluctant to talk about, which occurred long before they took part in the history of the Elves. Not to mention the Maiar Orc commanders theory; the Boldog.
Another example; the Prophecy of the Dagor Dagorath. The final battle between Melkor returned from the void and the host of Iluvatar. In the earlier works it is a resurrected Turin who finally destroys Melkor, but in later works this final battle doesn't occur at all and is a different battle.
What adds to the confusion is a certain group of people who only consider the following works to be valid as they were published during Tolkien's lifetime:
The Hobbit
Farmer Giles of Ham
The Lord of the Rings
The Adventures of Tom Bombadil
Tree and Leaf
Smith of Wootten Major
These purists write-off The Silmarillion and The Unfinished Tales as merely fan fiction.
Welcome to the fractured world of Tolkien scholarly.