Relias and a couple of the other champions have access to spells that do damage based on their level. Going from level 11 to level 16 is effectively a 50% increase in their ability to deal damage, as long as you have enough mana stockpiled to allow for a battle or two with active spellcasters (which you might not always have). Also, while there are up to six (seven with Path of the Mage) experience-increasing traits taking away from your more useful skills, these more useful skills don't do anything for you if you cannot hit the enemy or die before hitting the enemy. And since you're stuck with a mostly-champion or all-champion stack for much of the campaign, you need to boost their ability to take and deal damage. Extra levels do that for you, and going from level 11 to level 16 gives you about 27% extra health without any health bonuses, or about 33% extra health with Path of the Defender (and more if you take the Endurance line of traits).
That being said, yes you should take really great traits if they happen to pop up. I would normally take Evoker and perhaps Prodigy over any of the Potential line of traits, and perhaps over the Trainer traits, but not over Knowledge unless I already had a significant experience boost in the campaign. Also, I would value Trainer I/II/III over Potential I/II/III for any unit aside from the primary melee tank(s) and primary caster(s), because Trainer gives everyone in the army an experience bonus which stacks for every champion that has Trainer.
