I found this interesting site that seems to confirm exactly the notion above, that the combination of string height (action), weight, and desire for accurate intonation (i.e. frets placed by ear to sound "sweeter") require frets to be placed out-of-parallel.
Check out the wild fret shapes in the photos..http://www.truetemperament.com/site/index.php
And more info in the FAQ section..http://www.truetemperament.com/site/ind ... go=4&sgo=0
"What�s wrong with straight frets?
Standard equal tempered fret spacing is calculated from one single piece of information about the instrument - the scale length (the theoretical speaking length of the open strings). A divisor constant is used to determine the locations of the frets. The scale length divided by the constant gives the position of the first fret. The remaining length after subtracting the first fret, divided by the same constant, gives the position of the second fret, and so on.
The model assumes an "ideal" or "perfect" string - one which only exists in theory, not in the real world. It assumes, firstly, that the strings have no stiffness. Secondly, it assumes that all strings behave identically, regardless of their thickness, whether they are plain or wound, and the material they are made of. Thirdly, it assumes zero string height - and completely ignores what happens when the strings are pressed down on the frets!
The strings themselves vary considerably in diameter and construction (plain or wound), and thus react differently to being fretted. One single adjustment per string at the bridge ("intonation") cannot possibly fully compensate for all these parameters at once, as they all vary in different degrees on different strings.
The only way to fully compensate for all these parameters is to adjust each and every string-to-fret contact point on the fingerboard separately, until each and every note plays the target frequency exactly. This, which is impossible on a guitar with traditional, one-piece, straight frets, is exactly what we do with Dynamic Intonation�, and Curved Frets�.