In a lot of T3 content, even in shit as low as like aArmored Nacrosena, it's insane how much difference/how smooth a good support makes runs compared to a 4th DPS. Sure, if you get lucky and get some omega shielding like an Arti and Gunlancer (let's be honest, they are practically a support anyways) it isn't too different. But, you have to play soooooo fuckin' careful without supports it's unreal and you don't know what you're missing until it's gone.
I main a Paladin so I never really realized, but going through T3 hards and shit now on my Arti and it's crazy how much of a difference they make. Most of the time I get a 4th DPS and I want to just immediately end the run, because I know it's going to be some braindead fuckin' Zerker or Sorc.