Not quite. It was mostly to get the combination of top tube and head tube lengths that I wanted, which are hard to find on stock geometry frames. Being able to specify a slack STA was sort of the icing on the cake. Most stock frames in my size have 73-73.5 degree STA, which I can make work with a 30-35mm setback seatpost. On my custom frames I run a 72.5 degree STA with a 25mm setback seatpost / ISP topper.

Plus it didn't hurt the custom frames were very affordable (another satisfied Tsunami owner here).