It's hard to quote exactly from your comments to address several issues and questions addressed in multiple posts. So let me just make some points:
1. RTF is NOT a panacea and I'm not suggesting that it replace LRF in any place where LRF works OK. But I think it is easy to get locked into "LRF for everything" and not consider other formats where they would work better. For instance, see my two posts:
https://www.mobileread.com/forums/showthread.php?t=24922
https://www.mobileread.com/forums/showthread.php?t=25092
I like this formatting (personal taste perhaps) and doing this with RTF is fairly easy.
2. I have read innumerable complaints from book creators and book readers that certain things (poems especially) just come out awful in LRF. Consider my new post:
https://www.mobileread.com/forums/showthread.php?t=25142
This is easily the best looking poem I have seen on my Sony Reader. There are no other tools known to me that allow me to do this.
3. I am not generally suggesting "converting" from other structured formats to RTF. I think that would be painful and probably counterproductive. But for creating new documents (see my first two threads listed above), it could be the easiest for a lot of things. And if you have "plain text" downloaded from Gutenberg or copied off a web page, then pasting that text into MS-Word and working with it may be easier than other tools you have available. That is what I did with the Keats poem.
4. Agree that RTF does not support links and other things that are important for a lot of documents.
5. I'm pretty new to book making, so I don't know all the things RTF will or will not let me do. I'm learning as I go along. Pretty much everything I know is wrapped up in the examples I mentioned.
6. Converting FROM somthing else TO RTF may cause the RTF file to be relatively big. Like HTML, RTF has the option of having lots of "hidden" secondary stuff in the file. I've had pretty good luck in the size being small when creating content, as mentioned above. Furthermore, if you know what you are doing, you can edit the RTF with a text editor such as Notepad and eliminate a lot of this fluff.
My bottom line is that after trying to get other tools to do some formatting I wanted to do, I found it hard or impossible. RTF allowed me to achieve the results I wanted. So at least for me - RTF is no longer a second class citizen, but a viable first-tier tool that should be considered as one embarks on each book making project.