Don't get me wrong, joining a match that's in its final round can be beneficial from time to time, but more often than not it's atrocious.
Most every time I start a match for any game mode other than duel, it wants to put me into a match that is already at its end, and more often than not, it's smack into a defeat scenario. More often than not, I replace a bot who has already suffered permadeath either by a pitfall or execution with no opportunity for a revive, or join a dominion match full of AFK farmers with the score already 800+ for the opposing team.
By allowing us to turn off the Join In Progress setting, we can be matched up with ACTIVE players in a fresh game where our victories or defeats are actually a result of our own efforts.