"Sidelined" is a POV based on a lack of context. Sure, Wayne would be sidelined as a character if Burton had no interest in him, but anyone who tries to digest the making of the movie at all would know making Batman minimal in the narrative was a conscious choice by Burton to portray him accurately (in his eyes). Whether it works for someone is another matter, but Burton honestly feels that Batman is supposed to be (and is better served by being) mysterious. It's an artistic choice, not an oversight.
The other matter is, of course, that Burton crafts his narratives to unfold with a lot of subtext. He likes just implying a lot of things without confirming. While I think this is a natural part of art (how many paintings use text to convey their intent?), I concede that it certainly made his Batman films open to negative interpretations. A catch 22, I suppose. While someone can look at Batman Returns and find a portrait of Bruce Wayne being reflective of his city and it's inhabitants (The Cat, The Penguin and Schrek are basically Wayne, tri-sected), others can come away from the film feeling like Batman isn't there.
