I live in Phoenix and I would definitely go to MLS games in the mid summer, although they would have to be at night. April & May games would be fine during the afternoon. June July & August would have to be at night. The usual temps in hot summer months, like right now are about 95 deg. at 8:00 pm, which is do able because its a dry heat. It would be easier to play in than a hot afternoon game in Dallas/ miami. The AZ cardinals are supposed to have a new retractable roof stadia built, but I don't think MLS wants to go that route. Unfortunately, I think my only MLS action is going to be driving to LA. I will keep hoping though.
My wife's family is from the Texas Panhandle (the area around Amarillo and Lubbock) and my wife went to college in that part of the state. It's almost as dry there as in the desert. Honestly, in the summertime, it's not too incredibly bad. Yeah, it's hot, but since it's so much drier than Dallas, it feels about 10 degrees cooler than what I'm accustomed. Plus, when the sun goes down, the temperature drops quickly because there's no humidity to hold the heat. I wouldn't recommend playing any afternoon games in Phoenix, but night games would work pretty well. And oh yeah, because Arizona doesn't observe Daylight Savings Time, sunrise and sunset are an hour earlier there than in most other places. Tonight, the sunset in Phoenix is 7:29 (according to weather.com). In Dallas, sunset is at 8:26 tonight. So for a 7:30 game in Phoenix, you'd get most, if not all the game at night, when the weather is most pleasant. Finally, another quick comparison: At 9 PM in Phoenix tonight, the temperature is supposed to be 95 degrees, with a heat index of 95. Meanwhile, at 9 PM in Dallas tonight, the temperature is supposed to be 90 degrees, with a heat index of 95. It could work.