I don't mean to discredit Opera Mini, and I did give it credit for platform compatibility and maybe raw speed. There are numerous reasons the Chrome solution is better for modern, web apps. I can't say for sure about speed, but anecdotal evidence and an educated guess would have me believe talking through a more local proxy and with SPDY it outweighing Mini's "binary" protocol.
Of course, with the web moving towards HTTPS, technologies like SPDY are forward thinking and proxies get shelved.
Of course, you are right, but for now most of the phones on the planet do not support "modern" browsers or have access to high speed internet.
As the other commenter outlined, for these kind of devices Opera is a keyhole to the internet.
And you will notice that even on modern hardware with good access to the internet, Opera works smoother/faster than other browsers (it's at least the case on my android tablet).
Of course, with the web moving towards HTTPS, technologies like SPDY are forward thinking and proxies get shelved.