I have on several occasions now clicked the "Join in progress party" and been matched with a duty that didn't have all of the roles filled.

As a DPS, it joined me in a group with only two DPS, no tank, no healer.
As a healer, it joined me in a group with no tank.
As a tank, it joined me in a group with a missing DPS.

With a duty finder that will take you out of the queue and put you into an invalid group, there is no incentive to click the "Join in progress party" option.