In my frame size, the STA can vary from 73 to 75 degrees and that makes a 2cm difference in reach. If I ignored the STA, I could end up needing a stubby 90mm stem instead of my normal 110mm. I want my stem to be a certain length and I don't want a high rise stem or a lot of spacers, so stack is also just as important to me. Sure you can make a poorly sized frame fit, but it will look like the buyer didn't know what he was doing.