I've always turned off Keyboard Clicks, Lock Sounds, and New Mail. Otherwise it would be annoying, for me with my own phone and for anyone around me I'm sure. I also turned off Keyboard Clicks on my wife's phone because it annoyed me. She didn't notice or complain.
In fact, I was just at a restaurant several hours ago, and there was someone whose phone was emanating all three of those sounds frequently, and I thought "oh, there's someone with an iPhone who doesn't know how to turn off all those annoying sounds"
I don't think Keyboard Clicks is a necessary evil. I can manage 30WPM without it on (and that's without autocomplete, the app I used to measure it didn't do autocomplete), and turning it on doesn't increase my speed. I don't even look at the keyboard while I'm typing. I look at the output only (though see the keyboard in my peripheral vision). I imagine many seasoned iPhone users do this, and do it with Keyboard Clicks off. I'm just saying, I think you just haven't practiced enough with Keyboard Clicks off.
I haven't been a library in ages, but shouldn't you be putting your iPhone on mute/vibrate in the first place?