You did read the books did you? So you know yourself. Not like I'm into doing a typical Xant discussion. Imo he's an important side character.
If I knew myself I wouldn't have asked you, would I? Edumacate me, tell me how he's as important as Daario, who
is Daenerys' romantic interest and a lot of pages are filled with Daenerys fantasizing about him, brings over the Second Sons to Daenerys's side which influences a major battle a lot (after killing their two other leaders), and is the leader of a said big mercenary company the whole time, becomes an ambassador to Lhazar on behalf of Daenerys, and then ends up as a hostage, which is significant because he's Daenerys' lover.. in other words, he's a big part of Daenerys chapters.
To the left, we have Strong Belwas, who is a fat dude who likes to fight. His only real deed in the books is fighting the duel and Daario was just as suitable for it. There is literally nothing he does that cannot be done by someone else. Same cannot be said for Daario.
So remind me, how are these two as important?