One thing I've noticed with LRF over other formats on the Sony is that the files are generally smaller, faster and have somewhat more predictable formatting than just dropping the RTF directly in. But even with that, I still keep the RTF even if I make a converted LRF simply because it's good to have options later.

Best thing about HTML is that you can convert that to any other format. It makes an excellent base for conversion even if your current reader doesn't handle it and the basic format itself is an easily worked and known variable for all other formats.