Defenders and assassins are not worthless they can both be made unkillable fairly easily. Defenders can get a massive amount of health, especially ironeers, and reduce damage from most troops to pretty much nothing. Assassins never get hit, period, make it a wraith and they can solo the map on auto-resolve.
A level 20ish defender with a lightning hammer can actually put out some OK damage, with other good gear they can do like 50 damage per hit, which can outright kill a lot of the crappy troops the AI sends at you (even companies), and militia. Armored targets, with levels and health bonuses, a crushing blow will make a noticeable impact on. It's not great damage obviously, but it can help, and their main job is to be in the middle of the enemy absorbing blows.
Assassins can pretty easily do double that damage, built right they can almost do as much damage as a company. Again while being impossible to kill.
Depending on how the Reap change in .80 works warriors might also become useful, if it becomes 10 health gained per hit they might be able to hold their own in a fight with the right equipment. I haven't tried their overpower skill yet, I'm guessing warriors can destroy pretty much any enemy troop, no matter how well geared or leveled. More damage to trained troops than assassins, but less damage to single targets.
All that is ignoring the tactical spells most of them can cast, regardless of being a mage or not. Also keep in mind we are still playing with mostly FE champions, the better ones have not been updated to LH yet, we'll see next patch how they do. Just look at the spider guy champion new with LH, he comes with a +5 health per level trait, that's huge. I've been hoping to get the ghost champion, I'm guessing she is immune to physical damage, which is basically game changing.