I genuinely believe that if he were eliminated it would likewise eliminate alot of the exposition of Yuna's character (who IS detrimental to the story). That's what character foils are for - think Cloud to Zack, Seifer to Squall, Brahne to Garnet etc.
For me, Seymour seemed to be the opposite of Yuna, like a ying vs yang type thing.
Yuna - half Al Bhed = Non exceptance.
Seymour - Half Guado = Non exceptance.
Yuna - Daughter of a High Summoner.
Seymour - Son of a Maester.
Yuna - Trying to ease Spira's suffering by defeating Sin. (bringing the calm)
Seymour - Aiming to ease Spira's suffering by becomming Sin. (ending life)
You could probably find more opposites/similarities.
So for Yuna to show the good, Seymour is showing the bad. Thus bringing forward Yuna's character more and making her look stronger.