Stack really does have some value to it. It totally eliminates fork length and BB drop from the comparison. If all you do is compare the head tube length, then BB drop and fork length differences could lead to unanticipated changes to the spacers required under the stem or the stem angle, to get the desired setup. Headset height still needs to be considered, since some frames may have a 20mm minimum headset top section and another may allow as little as 8mm.

The idea behind reach is to eliminate the seat tube angle from the fit comparison, which it does, but I've never seen a manufacturer explain that reach comparisons are only valid at ONE stack height. You can find frames that differ by 20mm in stack height and list the same reach, but they will not fit the same, when the handlebar height is set to the same distance above the BB. The "handlebar reach" of the smaller frame will be about 6mm shorter, with the same stem.

Measuring the horizontal ("effective") TT lengths is fine, but a buyer has to understand how to correct for any difference in the STA, in order to figure the true difference in reach. Many buyers don't understand this and only compare TT lengths.