If you do a good EPUB you can generate a good MOBI so there is no need to submit those separately.
Not sure I understand the claim being made here. If you are claiming that given a good EPUB you can just run that through Kindlegen and generate a good MOBI, then that is certainly not true, and the existing PG EPUB and MOBI book postings give many examples of this. There are many PG EPUB generations that end up looking pretty good, but the MOBI, which is more-or-less generated from the EPUB [1] does not look good. Conversely, there are some cases where PG generates an EPUB and then generates the MOBI [1] from that, and it ends up the MOBI looks good, but the EPUB does not. [1] No exactly true, since Marcello makes a somewhat-custom-version of the EPUB used to generate the MOBI from that version of EPUB via Kindlegen, but the simplified principal expressed is more-or-less the same. Now the reality of my "work-chain" -- which essentially consists of trying to push a rope from the wrong end -- is that I try to create a good MOBI file, but conceptually I do that by trying to understand how Kindlegen creates a MOBI from an EPUB, and I try to understand how Marcello tweaks the EPUB that is generated specifically to be converted into MOBI so that I can create a good MOBI, but to generate a good EPUB then I have to understand how Marcello creates an EPUB from an HTML so that I can create a good HTML to make a good EPUB to make a good MOBI. And I test all this stuff before I send it in to PG with my own local copy of the sausagemaker software. And then I send my stuff into PG, get yelled at for a while until someone at PG actually decides they might actually want the book, PG posts it, and then surprise, what PG posts is not entirely what I expected from my own local tests. Why try to make a good MOBI in the first place? Because in my experience if I can get the far end of sausagemaker to "work" then the intermediate stage of EPUB and the primary stage of HTML is pretty easy to get to work. And the txt70? God only knows. That part really doesn't fit into my "work-chain." I make it last, and extremely reluctantly, as a reluctant precondition to submission. And because neither I nor anyone else apparently has any decent tools to make the txt70 format. And then the WW'ers howl because they still pray at the altar of txt70. But I am so not there.