However, I've always viewed the Second Age if not as "filler" than certainly as bridging material between the other two ages, which Tolkien was much more preoccupied with. So TV works for the Second Age, to me.
That would very much depend upon what 'arc' of the Second Age you're referring to, it being a 'composite' of two quite separate works: Of the Rings of Power
(the forging of the rings) which is basically just essential backstory for the events of the Third Age War of the Ring
The latter is certainly not 'filler' or 'bridging' but in conception was a wholly distinct pre-LotR work penned by Tolkien in the mid-1930s, originally forming part of a pre-publication of the Hobbit science-fiction time travel novel called The Lost Road
(1936), in which a philologist has recurring dream encounters with 'Elendil' the Númenórean (survivor of the Anandune / Atlantis sinking) and eventually comes to realize that he is the 'reincarnation' (in some sense) of this ancient pre-Ice Age personage and can even 'remember' his life experiences in the dream-world. So he returns to Númenor in thought before the deluge, as Elendil speaking to his son Herendil (Isildur), when it is in the process of being corrupted by the evil divine being Sauron (appearing here for the first time!) under its tyrannical King Tar-Calion (Ar-Pharazon); where Tolkien describes in detail for the first time the satanic cult that has taken over the Isle under its high priest Sauron in his dark circular temple in Armenelos, the impending war with the Valar to wrest eternal life from the Undying Lands, the persecution flight of the Faithful etc.
Númenor, therefore, is not some filler-appendage between ages but a proper full-sized pre-LotR and indeed pre-Hobbit conception in itself that Tolkien continually returned to and progressively developed throughout the 1930s-1970s (in the Lost Road, The Notion Club Papers, The Drowning of the Anadune in the 1940s, The Akallabeth, Aldarion and Erendis near the end of his life etc.).
Indeed, it was when he couldn't get his time-travel Númenor-Atlantis novel published after the Hobbit in 1937, along with the Quenta Silmarillion, that Tolkien started (reluctantly) on LotR - another story about Hobbits which was what his publisher wanted (whereas he wanted to write about the Silmarils, the Noldor and the Númenóreans).
But Tolkien later found a way of 'synchronizing' Númenor into this new Hobbit-tale by species-bending Strider (originally a Hobbit named Trotter!) into none other than Aragorn, the Númenórean heir of the line of Elendil; with the 'bridging' Celebrimbor material ultimately being created to connect Númenor-Atlantis with the new forging of the rings plot.
That Númenor originally bore no relation to the 'rings of power' is evidenced by the fact that Tolkien never really worked out how the 'one ring' survived the Deluge when Sauron's body was destroyed but his spirit fled back to Middle-earth to reincarnate itself. In the original Drowning of the Anadune, Sauron hadn't had a Ring because there weren't yet any 'rings of power' at that time!
(This post was edited by Althoun on Jul 12, 10:10pm)