I live in Phoenix, AZ, so heat is a bigger concern than cold. For commuting to work, I take the truck when I know if going to be over 106. It is just no fun over 106 with full gear in stop & go traffic. I am an AGATT rider, so I really get sweaty on my work commute, even with my Motoport Kevlar mesh suit, . I'll also avoid riding if I know there is going to be rain, which is all of 10 days a year in AZ.
Now as far as cold, if it starts out the day in the 30's, I'll take the truck. Luckily it is only a few weeks out of year when it is that cold. But as long as it is 40's or warmer, I take my bike.
Now the above applies to my work commute. If I am riding for fun, I will ride through 110 plus degree weather to get to nicer stuff. There are a ton of cooler areas 50 to 100 miles around Phoenix that are really nice to ride when it is summer. Sadly, when it is nice in Phoenix, those areas get a tad cold.