Have you heard of A/B testing? The internet is amazing. Since there is very little real cost to displaying websites (as opposed to billboards, or other forms of visual marketing) you can do A/B testing and statistically know if your change is going to be better.
In life I wish I could test things this way. Imagine if I could A/B Test my recent car choice. I could know through real data which car would fill my requirements better. Web designers have a real tool in their arsenal, and they should be using it a lot! For me, I'll just have to rent the other vehicles and see if I still think I made the right choice.