We always size a frame according to its "reach measurement" too, simply because we know we prefer running 100-110mm stems. Knowing the frames reach makes proper fitting way-easy.

Look at it this way:
If you built three 54.5cm / 150mm headtube frames - one with a 73-degree seat-tube, another with a 73.5 and the third, with a 74-degree tube; each frames reach will differ quite a bit even though each is considered a size 54.5cm, perhaps forcing you to run an "other" length stem.

That's why many fitters still prefer the frame "reach", rather then "ETT" alone. "ETT's" still a neccessary factor, though.

It would be great if every "off-the-shelf" bike company would include it in their geometry specs - so much easier.