What's wrong with tailoring the sound?

Probably been addressed a bazillion times but I'm wondering why it is apparently so wrong to tailor the sound with tone controls? I read lots of posts on the various audio forums and hear things like "these speakers may be a tad shy in the bass but...." So whats so wrong about having a devise which will aleviate this problem in an otherwise wonderful speaker? Won't this increase the listeners enjoyment? I also read about certain cables being brighter or darker than others. It seems that the only way for this to be true is if certain frequencies are being altered in some way. Why spend hundreds or thousands of dollars on a cable to take some brightness away when one could just tweak a knob and get the same results in a few seconds (and for free) What is to be done with recordings that aren't quite up to par? The overly bright or overly dark. Should they just be discarded in favor of audiophile quality recordings, content be dammed? What do you do when you want a little more depth in your sound when the lack of it is due to unavoidable room conditions? Are there good quality units out there that will allow me to have hi-end sound AND be able to adjust/compensate for personnal preference? The recording engineer did not mix the recording using my equipment, in my room, using my ears and with my personnel taste in what I find most pleasing.
I've also read enough posts on these forums (though not usually on Audiogon) that I will ask this favor. Don't beat me up too bad, I'm relatively new.