There are times when I'm tired talking about .epub and .mobi. The problem with these formats is that they tend to be US/UK-centric. Take for example .mobi*. While it was semi-popular back when people still carried PDAs, it currently thrives solely due to the existence of the Kindle and the Amazon ecosystem. Now the problem with the Kindle and Amazon is that it mainly caters to the US/UK crowd. How many Southeast Asian countries for example have Kindles sold at local retailers? Or for that matter, will online retailers actually sell them eBooks (due to geo-restrictions)?
Contrary to popular belief, eBooks aren't a new concept. You don't need an eBook reader to read eBooks. The first eBook was born when word processors were developed. If you ask a friend to send you a copy of their draft, whether novel or short story, they'll most likely send you a .doc or .rtf file (or .odt depending on their word processor). I once got sent a .pdf but that's the exception. Unless a commercial transaction is involved, I doubt that they'll be sending you a .epub or .mobi file. The problem with the eBook hype for the past few years is that people, especially Westerners, assume that eBooks must be .epub or .mobi since that's what the major vendors are selling. But in reality, various eBooks were already circulating even in the days before Amazon or iPods, whether they took the form of the lowly .txt or .pdf file.
If you look at the pirate economy, majority of the files aren't .epub or .mobi (although that's starting to change as newer uploads are starting to use those files) but .txt (because it's simple), .doc (because it's common for users to have word processors), .pdf (easiest output for scanned files, especially books which have images), and .lit (a remnant of the PDA generation of eBook pioneers). These formats are being circulated around the web and easily the format of choice for those without commercial eBook reading devices. To me, this is an interesting phenomenon because it sidesteps the ecosystem of various distributors (Amazon, Apple, Sony, etc.).
Similarly, related to this is the underground hardware economy. One of the local eBook readers (my review here) available in the Philippines (but sourced from China) doesn't adequately support .epub (it won't load in my experience) but among the other files it supports is .txt, .pdf, .html, and .fb2 (according to Wikipedia, a format popular in Russia). It's newer brethren has a similarly eccentric list of supported formats. Here's the specs of Korea's Soribook (no .epub or .mobi).
The problem with .epub and .mobi is that it's honestly optimized for one type of book (fiction/non-fiction books without images) and this design choice is reflective of the culture that's pushing it. Take for example Japan, home of the cellphone novel. It's a country where manga (or comics) comprises a significant chunk of its publishing industry, currently comprising as much as 25 percent of book sales as of 2006. Any intelligent book designer will tell you .epub and .mobi is a poor fit for comics (and why there's a separate app for comics on the iPhone/iPad Touch, such as ComiXology). It's even more shocking to find out that "83 percent of all e-books sold over Japanese cell phones, were digital comics". Even disregarding Japan's manga phenomenon, there is also the practice of reading right to left, top to bottom (left-to-right is practiced as well but it's usually reserved for titles and short phrases) and that's simply not supported by either format. Instead, while Japan is waiting to create its own format, what's currently being used is .xmdf (for both fiction and comics) and .pdf. China has a similar challenge as it's home to various formats which I've never heard of before, such as .ceb and .pdg (and I'm not yet tackling Korea). The prominence of the mobile phone as the eBook reader of choice also steers the entire eBook ecosystem in a different direction. One of the factors is the fast broadband available in countries like Japan and Korea but another is how the language itself changes the way that culture reads; for example, you can read an old essay of mine which explains why Japanese readers read fast, and why their language occupies less characters on a screen: Japan Ain't Necessarily A Nation of Speed Readers.
There are two factors that influence the adoption of another culture's technology. One is the tech that's currently available. For example, in the Philippines, while HD TVs are available, virtually no vendors--not even pirates--are selling Blu-ray discs and players (the exception are Playstation 3's and laptops with built-in Blue-ray drives). DVDs are still the format of choice among consumers and pirates. I was listening to an interview with South African author Lauren Beukes and one of the phenomenons in her country that she mentions is how South Africa simply skipped mp3s and went directly from CDs to music played on cellphones because the latter had a huge adoption rate. The other factor, and this might be a chicken-or-the-egg dilemma, is the culture's awareness of the said technology. In one of Salon Futura's podcasts, Alisa Krasnostein, a publisher from Australia, mentions how reviewers don't know what to do with .epub files (as opposed to .pdf), even if you don't need to have a portable eBook reading device to read them. It's not an issue of technology per se (as .epub can be read on the PC, assuming you have the right software) but a general ignorance of the file format, how to use it, etc. Here in the Philippines, except for a minority of readers (which includes myself), most people who read eBooks aren't that familiar with .mobi or .epub. (Similarly, one of the problems I encountered was finding a good .epub designer; it's easy to look for graphic artist who knows how to layout .pdfs but that's not the case with .epub.)
Now I'm not advocating the abandonment of .epub or .mobi files but readers should be aware how imperialist the said formats can be, and the technological divide separating them from readers of other countries. .epub and .mobi are formats optimized for a specific culture and the technological innovations and ecosystems built around them favors readers of a specific demographic. Readers who use formats like .txt or .doc might look primitive compared to early adopters of other eBook formats but is that really a fair comparison? What gets lost in a lot of discussions is that it assumes .epub and .mobi are the superior formats and should be adopted by everyone when that shouldn't be the case (at the very least, it's not the scenario for Japan and that's a country with a lot of paying readers).
*.epub is more widespread than .mobi.