I think it's up to each person to decide. I mean it entirely depends on why you play the piano, what your goals are, what's important to you, right?Like if you are an amateur who's just playing for fun, and you like the 2nd movement of the Pathetique more than the others, or that's the only one that's within your graph, why would it be wrong if you just played the 2nd movement?I think in the case of sonatas, in many cases they are constructed so that the movements form a narrative, and therefore playing just one movement is like reading just one chapter. If you are an amateur just playing for yourself, why not read your favorite chapter over and over if that's what you like? If you are a professional who intends to have the "job" of presenting the narrative of the work to the public, then you play all the movements.At the same time, I don't think there is anything wrong to select just one movement to play as an encore.I also think if you are a beginner/intermediate student it's fine to study just one movement that you like or will benefit you, and when you get to advanced level it's good to study the whole work since part of the craft you are developing at that point is to build a narrative and playing a wide variety of characters of music.Just my opinion though. I hope this makes sense.