Is he not an upgrade over Thetis or Luin?
Luin is a wood type and each wood unit adds -5% DEF to Navi's overgrowth (for a maximum of 10 wood units = -50% DEF(-70% DEF if ignited)). And besides debuff duration Thetis can also apply poison. I guess if you already have 10 wood units without Luin you could use Seth for non-stop debuff duration since taps don't have cooldowns. And/or if you have a better poison unit than Thetis. Thankfully you can do test runs in WB and taps aren't that expensive to max so you can easily gauge if he's an upgrade.