Leaf's crimes Edit
You see, leaf was responsible for Clara's parents fucking and thus setting into motion the events of Series 7, 8 and 9, undoubtedly some of the worst seasons the show has ever had. It is essentially responsible for everything wrong with the Moffat era.
Leaf is secretly a Doctor Who hater, and it wanted to bring back the negative reception the show had in the 80s by introducing a character somehow more annoying than Peri and Mel combined. I think we can all agree it succeeded.
Leaf was capable of super strength. By striking him in the face, it was able to knock Clara's dad from the sidewalk and into the road, almost getting him hit by a car and making him look like a complete retard in the process, like he forgot to SPLINK or something. It could also cause anyone associated with it to begin making pretentious dialogue, which explains about 99% of Clara's lines in the show.
Will it ever return? Edit
Let's be honest here, it's achieved its mission. The damage to the show is done, and unless Doctor Who manages to regain its reputation of the best show ever like in the RTD era, I doubt leaf will ever come back to face off against the Doctor again.