I keep mine almost all the way up. Why would you want to dim it?
Because it considerably affects battery life!
No offense whatsoever intended to the poster, but you keep hearing "oh, my 4s gets terrible battery life"...and then you dig a little deeper and find out they have brightness cranked all the way up, they actively use the device for hours with the screen lit, they never turn off Bluetooth (even if they rarely use BT devices), and they have like 40 apps set up for push notifications and using the GPS. All of these things use power!
If you prefer your device on full blast, that's perfectly fine, but there's a trade off for that. If it didn't take power to run the screen, then Apple wouldn't even have bothered to ever dim the screen or have it go to "sleep". It would just be perpetually "on", kind of like a Black-and-White Kindle or something, and run for a month. This is a much more complicated device.
Mine is probably around 35%, or near the "B" in many of the pictures above. I actually would prefer to have it around 50% (at least in well-lit areas), but the way I have it set up right now I've been very happy with the battery life, so I'm keeping it.