As far as roles in a party, monks aren't the best tank, aren't the best striker/damage dealer, aren't the best controller, aren't the best healer/buffer.

The one role that monks are supposed to be the best at, skirmisher which by the way is more of a play style than "role", rogues are better at by a mile. Ranged attack with bonus action hide beats the pants off any cute movement bonuses monks get (which rogues get as well). And the rogues offensive ability (sneak attack) does not conflict with their defensive ability (bonus action hide) unlike the Monk who is forced to choose.

And even the tertiary roles such as scout and skill monkey, monks aren't the best. So to the OP, I agree the monk's role is as a 5th member when all the roles are already taken because they aren't the best at anything. On a positive note, they are versatile enough to fill many of the roles somewhat adequately, although bard probably has them beat in this regard as well.