Summary of reasons why Seth is good:
1. He's got higher total growth rates than every other potential paladin in the game, by at least 25%.
2. He's got respectable starting stats for a level 1 Paladin. They're higher than FE7 Marcus', which was already significantly higher than most Jeigans (see FE6 Marcus <_<).
3. Seth isn't as vulnerable to being RNG-screwed as all the other cavaliers. This is considered a big plus for experienced gamers.
4. He starts off with both A-level weapon ranks that consist of Swords and Lance.
5. Because he starts as a Paladin, you just saved yourself a Knight's Crest and hundreds of exp if you use him instead of another cavalier.
6. Excellent support options. Eirika, Cormag, and Natasha to name a few. Anima is also a good support element which is better than Kyle's Ice affinity as well as Amelia's Thunder affinity, IMHO.
7. Would you believe that on average, Seth as a level 20 Paladin will have a higher stat total than FE7's Sain at 20 Paladin? Because it's true.
It's true that most Jeigans aren't great. But Seth was a really weird exception. Why? Because his level up stat growths rates were phenominal. Believe it or not, they were actually higher than every other potential paladin's.