With Liepard it doesn't really matter either way because it's never really used for attacking, it's used for its ability Prankster. Also nearly all Pokemon can learn a mix of special/ physical moves, probably to make things easier in-game, since there are Pokemon that have really high defence or sp. defence stats (or both, like Shuckle), so imo whoever designed movesets just wanted to avoid more inexperienced players getting stuck when they have Pokemon with only physical moves.
In the case of Toxicroak, it's to give it diversity... I mean that's really the point of Pokemon that trainers can use whatever Pokemon in whatever way they deem viable, and special Toxicroak is actually really good for a surprise element.