@gennoson
I do get your point. Indeed being oneshot by monsters would make healers less useful. In addition to that, a heal that wouldn't regrow limbs would lose a lot of utility. But that's only for healer. Casting a speed buff, even small, on a fighter, would be a huge improvement in a life-or-death situation. Even more in those hardcore settings, where a 0.2s faster reaction would save many adventurer's lives.
To use the same example as you did, that old man. Supposing that, before that fight some "talkers" (cmon that class name is lame) had casted an intermediate speed/reaction speed buff, then,
maybe, that old man would have lived. That buff could have granted him enough reaction speed for him to avoid that one attack that ripped his body in 2. I'm using speed buff as an example, but any other buff could have made a difference : a defense buff would have prevented him from being too worn out from taking hits, hence allowing him to have enough stamina left to react in time against that one (again) critical attack that split his body in 2. An attack buff would have helped him kill the monsters faster, using less stamina this way, resulting in the same outcome... etc.
That kind of teamplay would require the buffers to cast the spells then retreat before the fight starts. But even so, those buffs would make a huge difference in those skirmishes, even more in hardcore mode where no mistake is allowed. Buffers may not be the optimal team mates depending on the world setting, but I honestly don't see why people would call them the weakest (unless it is REALLY biaised, as in requiring 1min chant while standing still for a 10s buff). There's also the fact that they could buff themselves to become as reliable as any other fighter, but that's not the point here.
But ye I'll wait for more updates on the class, your point of view is correct. It all depends on the world settings.