To be honest out of the four'roles' in D&D. (Tank, Healer, Arcane, Skill Monkey)
Really the only two that it gets harry without are healer and skill monkey.
And it's not like you can't be without those two.
I have played entire games with no trap finding character.
I have played entire games with no healer.
And that's much harder then playing with no tank or arcane caster.
To be honest arcane casters are the best 4th man, but also the least needed.
There is also a matter of being able to double up on roles to fill them.
A bard/wtich/paladin can cover the healing for a group without the need for a actual healer. (It' Might take two of them to manage it)
Wile a Ranger or Monk are not normally tanks a few good melee's can double up and pretty effectivly cover the tank role with other tactics.
I have discovered over the years, there is no reason to have a dedicated ranged combatant. (It can be nice)
Because honestly everyone should have a hanged weapon, and you can just mass fire any ranged problem. (That REALLY goes for arcane casters, you Should REALLY have a crossbow or sling)
And Acrance casters at there very best are utility character that can do good AoE damage.
Honestly speaking a Cleric Can be built to cover three out of the four roles of a group. (Everything but trap monkey) And there might be a domain for that.
As a matter of fact the most broken group I ever played in was a.
Was basically unstoppable.