While I agree in principle with you and chris, this isn't how it always works.
First, there isn't always someone at FOH (or someone who's paying that much attention). I've played many shows where the person sets levels at soundcheck and may or may not even stay at the board - much less actually noticing whether some specific part is at the right volume. Of course, touring bands and larger venues etc have their own dedicated soundman but many of us don't. Ever played in a local club where the "soundman" is also the bar-back? Half the time you can't find them even when you need them for something major.
Second, even when I dial it in *perfectly* at practice (and at gig volume), I'll get to a venue and there are many variables - stage layout, room acoustics, etc - that sometimes require the ability for quick boosts or cuts. It's great to have FOH who are committed to our sound and familiar enough with our songs that they know what to do, but playing in bars you often don't get that option - so it's up to us to fix on the fly. +/- is a great solution for this, and it was much easier to use on the II which doesn't have a dropout ;-)