But I have to say that this is about as clear and direct an explanation as you could wish for, and I find it convincing. This is from a Timbers Q&A with Merritt Paulson:
This is very simple. If we could have a quality grass field, we would. We cannot. PGE Park is below grade and only a small fraction of the field, especially after our new configuration, receives direct sunlight. That fact in and of itself makes PGE Park a unique challenge for grass even without the weather in the Pacific Northwest. In addition, add training needs (although we will provide a grass training complex for the team) to our fall high school and PSU football games that we will host, to our weather and you have an equation that simply does not work. Imagine an October soccer game, when the grass no longer grows in Portland, the day after a football game on grass. No way to get rid of football lines and an awful surface. Our organization has looked at this long and hard and will continue to look at it as technology advances. Turf is much better quality today than it was 5 years ago, and we will look at a product that favors soccer. Additionally, we will likely have our own custom grass cut-outs for exhibitions when necessary.
Let's hope that field turf comes up with a modified, soccer-specific, product soon. I think that the main adjustment would be to have slightly longer blades of fake grass to prevent the ball from bouncing too much and to slow the momentum of a rolling ball. But still, soccer played mostly without tackles is just not the same.