Our company sell to developing countries too, and there's definitely money to be made.
If you don't want to support people who you automatically assume have no money to spend, go ahead and leave it on the table for us to collect. :)
If a business doesn't care about users in developing countries, that's their call. I don't think either way is wrong but the front-end people should at least raise the issue and then go with what the client needs. So many websites target a relatively small geographic area that they can safely ignore Opera Mini users and continue to thrive.
Not really. They might be perusing your e.g. news site for information, but wouldn't click in any ad anyway (and maybe, because of Opera Mini, they don't even see them).
With the advent of the $30 smartphone, by the time these countries have disposable income that you can indirectly or directly profit from, its likely that one of their first purchases is a better phone with a capable browser. Until then they're a waste of time and resources.
On Black Friday you could get a Moto E, unlocked and off contract, for $10. This phone has a quad core processor, an LTE radio, a gig of ram, and a 5 megapixel camera. Ten dollars gets you something that can outperform flagships from 2 years ago.
But even putting that aside, you're ignoring the fact that as these developing nations develop and have money that's worth going after, bandwidth is certainly going to become cheaper as well.
Opera Mini doesn't render webpages, it's all done server side. What's sent to the browser is a pile of highly compressed layers containing text and images and some metadata about what's clickable - and that's it. From HTML, JS and CSS point of view Opera Mini is a highly optimized, non-real time remote desktop client.
I've heavily used J2ME Opera Mini on my S40 dumbphones many years ago. Back then it was awesome. Today, when the only way to keep sanity when browsing the web on desktop is using NoScript, I doubt that using it would be as awesome as it was back then, and that's not really Opera Mini fault. After all, when using Opera Mini, you don't expect everything to work and render as it would on desktop browser - as long as content is shown and can be accessed properly, it's fine. However, in a world where crazy people use SPA frameworks for their blogs, it cannot be taken for granted.