On the contrary, I still use RTF since it is much faster to generate for me. With LRF I always need to go through the book in Book Designer and correct the titles, subtitles, etc. It takes some time, and, personally, I don't need chapter headers, TOC, I just need the text
Yes, RTF needs more space in Reader, but if you delete the books you finished reading from time to time, the file size is not an issue.
Generally I would decide regarding the quality of the source file formatting. If it is good, RTF is clear winner for me, when it needs more adjusting, there is Book Designer at your service.