If you backed up a microphone 2 to 3 feet to pick up all of the holes in a flute or sax on a live gig, you would hear very little of the instrument in the microphone signal. This is because of the high amount of leakage present on stage. Even in a studio, loud instruments that are on the studio floor can prevent a microphone from picking up a sufficient amount of the intended instrument's sound. In situations like this, the engineer must realize that getting distinction is secondary to getting sound. For an acoustic guitar, placing the microphone at the hole gives a louder instrument signal.
More sound is gotten of the sax by cramming the microphone down the bell.
With these instruments, the board equalizer can be used to improve the tonal quality of the instrument. The piano, in particular, will pick up a lot of leakage. When micing a piano with the band it is best to bring the microphone to an inch or so away from the strings. This gives a less even tone, but again you have piano picked up. There is no chart that has ever been written which will tell you where to place a microphone for every session or for every gig.