Simp,
this is a good question. I think that it mostly has to do with the weather conditions. For example when the Giants were playing in arizona the other day, it was something like 115 degrees outside, so therefore they would close the roof, and get the air conditioning Pumping. I think they said it was around 80 inside, but can't remember for sure. They have to make the fans comfortable enough to watch the game. I saw that there were a few people having to sit in the sun at the BOB (Bank One Ballpark) durring that game. Now, I know everyone is thinking if there is a roof how could there be any sun? well, there are windows at the top of the stadium that allows sun light to come through. At least they had the air conditioning, it was funny though b/c people 5 seats away were in the shade, and those people although they thought it was safe inside were still getting sun. Glad I don't live in AZ, especially durring the summer. So, to answer the question, I think it has to do primarily with weather conditions. I know in seattle it rains a ton each year, so therefore the roof is handy. Off the top of my head I can think of 3 that have the retractable roof, SEA, MIL, and AZ I am almost positive there are more, juscan't think of any.
