When fighting off-element, Mordred is always an okay choice. As you've noticed, debuffs MATTER, and Mordred does a great job at helping those debuffs through. The ideal is to be strong enough to use Hercules for powering through content even when debuffs miss, but if you're fighting off-element, that might not be an option depending on content. You can use Mordred in those cases, and if you're fighting off-element, the offensive weapon is INVALUABLE due to giving you a massive boost to your Elemental attack.
Yes, Mordred's offensive weapon skill sucks ass (though I had a giggle using it in a Rage Bar- Water team), but that's just how the cookie crumbles. You don't care for the skill, you care for the Elemental attack.
At least on the good side, Mordred's Relic burst effect is great. Combo- is seriously undersold on these forums, and Mordred having access to it without using up the EX slot is amazing.