Andrew R Stewart 

1- Care because the overall system performance is becoming more and more dependent on best chain lines. If you're running older systems that have no lift pins, have a wide ring to ring center dimension, have long chain stays than the chainline is of less importance but still will come into if you push the limits.

2-Chain line amount (optimal spindle length) isn't measured with the chain anywhere. Chain line is a chainring and a cog set center to frame center dimension. Now one can do a visual check that tells a lot by placing the chain on the most centeral cog and ring and eyeball how the chain runs between the ft and rr.

3- Because since the advent of the MtB there has been a difference between road and MtB intended specs/hub widths. Also some cranksets have a spindle length best for traditional seat tube diameters and OS ones (often between steel and Al frames)

4- Why not determine the current crankset's chain line and how that worked for you before trying to "paint by numbers"? If the chainline was good just duplicate that spindle length. If at all possible it's better to use real life info before any published guidelines.

5- See the above. I have installed what i thought would be a good spindle length just to find out a slightly longer or shorter one would be best more than a few times. Andy