I totally agree with this. I think an interest in composition makes you more inclined to look at the music you're playing from two different angles - the pianistic and the compositional. I (if I may flatter myself by terming myself to some extent a composer) keep finding myself asking "why did he express it like this, not like this or this?" I really do suspect that some pianists look at a score as something to be replicated and never ask themselves "why?"
That's a good point. I was studying a book about Bach's 48 Preludes and Fugues and the book talked about how Bach as a younger man was looking at scores and rewriting the parts that he didn't like, or making various improvements. A shame that some pianists would just see playing as a simple job like that of a taxi driver. As long as you don't crash, you're fine
MAH's a peculiar phenomenon: I just don't warm to his playing and find it a bit dry and lacking in character. However I do quite like him as a minor composer; it seems like the character and humour lacking in his piano-playing emerges in his compositions.
Yes, much more known for his technique than for his interpretiations. I also find his compositions very well written and yes, at times humorous!