@The5thSeraph
Unless the game specifically makes melee or magic not effect an enemy, then how damage is dealt doesn't matter you are doing damage.
here lets take this as an example
one class does dmg over time, one class does up front spell damage, and one class does melee
If the mob doesn't resist magic, it doesn't resist melee, there is no weakness then all damage is the same, at the end of the day, its numbers going down
with tank, the only defining feature of a tank is the ability to hold aggro and live, if you cant do this, then you are not a tank, even a horrible tank, worst in the game, as long as it does this, is a tank.
now later on you said phantasy star or monster hunter... those aren't mmos, they may call themselves that, but I wouldn't call diablo an mmo either
when you go into mmo, there is one defining characteristic, and that is a large scale fight, as in you have more then lets say 12 people killing one monster. with an open contested area that could have hundreds of players, (east commonlands was the hub zone in everquest for example that everyone idled and sold crap in)
in my mmo of choice, everquest, we use to have open world raids, where you could bring as many people as you wanted, then we moved to instanced, which limited it to 72 players, and later on due to player counts going down they balanced raids around 54, and from here they balanced many raids to have lower player requirements but also more mechanics that would screw over the raid if not followed.
now in a raid, there are several ways they screw with players to make things harder than need be
a balanced fight, you have to fight 2+ enemies at the same time, keeping hp within a certain range, everquest its typically 5%, there may be a hard fail mechanic, fights over, or there may be a power up 'you did a wrong' that most guilds, if given the choice to balance or burn, they choose burn because its less of a pain in the ass.
you have hard scripting, this forces the players to follow a hard guide on what to do, and if failure happens, it's usually event over. these are the worst, because raids rarely have every player actually being good, there was a guild that would take new members to an old raid zone, something where you could solo the entire event, but the scripting made it impossible to do unless you follow it, it was their way to weed out incompetence
you have the raid spawning very high dps enemies or very high health enemies, these need to be rooted off, or they need to be stunned (everquest there is a mesmerize effect that is 1-5 minutes long stun that breaks when touched) because these either hit WAY to hard, or just have so much hp its not worth dealing with them, that said, many times they will be immune from root or stuns, so you now need to agro them, and run them around, more or less allowing the dps and tanks to do their job
you also have gear checks, these are events where if given time, you could brute force the event, but you dont have time. these come in the form of a large hp pool enemy that hits stupidly hard, and effectively kills tanks outright, without a mitigation skill going, you will die, problem is these skills are on long cooldowns, so you have to pass it off to another tank, BUT, if you have high enough gear, you can handle it with lesser skills, so this requires a pool of tanks that can mitigate it, which lets be honest, perfect pass offs and the like wont happen, so you now have to have good enough gear to deal with the enemy, they can also have large dmg aoe's. IO know there are a few in everquest that nearly out right kill people, but aoe heals are easily able to handle it, so its a gear check just to make sure you survive the event, or another type where the mob has stupidly large amounts of hp and you have to kill them in a set time, this being a dps check. these events are usually easy, but require a bare minimum skill amount along with weapons,
these are the various ways they screw with players, utility is just there to make dps dps, tanks tank, and healers heal better/easier/at all.
that's why it all comes down to a tank, a healer, and dps.
now in phantasy star or monster hunter, as many encounters are geared around 1-4 players, they can have various hard locks because, take an mmo foe example, they cant hard lock raid mobs because you now just made half a guild useless and are effectively hard gating content for their class, in a game where you can realistically spend over a year of real time building up said character. phantasy star, the one I have more time with and like, you can get to the harder areas in I believe about 60~ hours, so its not unreasonable to say play a different class.