Mystic Ally (especially Earth Ally) can be useful for absorbing ranged attacks (from those nasty wasps' bugs, hellflayers, shamans, etc.), drawing an elite or two away so you can focus down one of them, and acting as a scout/decoy so can get an idea of what nasty surprises up ahead.
They are also nice for blocking enemies in narrow spaces like doors and stairways, drawing elite spawned traps like arcane sentries away from you. Earth Ally gives you an extra 10% HP and can tank decently (taunts). I believe Earth Ally also scales with your gear.
Unfortunately, they aren't very smart and do things like scaring off treasure goblins and attacking invulnerable enemies.