If you go to a quality steak house type restaurant you'll always find salt and pepper on the table. The chefs cook the food to your specification, but as the eater you have the option to slightly season your meal.Onhwy61's analogy is excellent, and actually one that I use to encourage casual listeners to stop abusing the tone controls - i.e. a fine steak doesn't deserve to get smothered in sauce the way you might a ground sirloin from a truck-stop. I think that a prerequesite for the proper application of tone controls is that the system first sound pleasing to the listener without them, on the majority of recordings for which its used . . .
This is similar to good studio practice, which holds that the first priority (after making sure the instruments themselves sound like they're supposed to) is to use the best microphone for the application, and the second is to place it properly, before any processing or EQ is applied.
But there are a few problems with this idea that by adding any EQ or processing on playback, you're somehow getting away from the artists' "true intention" of the recording. For instance, there is a huge percentage of recording and mastering facilities out there that have rather idiosyncratic acoustics and monitor setups. Mixing and mastering sessions are also very frequently conducted at SPL levels that are way too high, and not at all representative of the playback volume . . . consequently, so many of the people making decisions about the final mix have significant hearing damage -- which I think explains the excessive upper-midrange energy that's present on so many rock and pop recordings.
So insisting that a flat EQ be used on playback, for "pureist" reasons, is IMO akin to insisting that Beethoven's metronome markings always be adhered to literally . . . it simply doesn't work in practice. And if one's musical taste extends significantly into the realm of sub-optimally produced recordings, a little seasoning can make a huge difference, just like an excellent reduction sauce on a sub-par piece of meat. Which just happens to be one of the foundations of fine French cooking . . .