Due to Probopasses massive defenses it is best suited for being a wall, supporter or staller, as big defenses are key to being effective. It also has Stealth Rock, Thunder Wave, Toxic and more which help it out a lot.
There is no way Probopass will ever be a sweeper in a million years. 40 speed and 75 special attack is horrible, and why waste those amazing defensive stats?
Hope I helped! :)