If it is warm enough outside and you elevate the core temperature sufficiently, there is no need to risk illness by wearing more clothes. So running with sweats on in July in Atlanta is not necessary (or wise).
I grew up in GA and once it got hot, there was no need to overdress, core temperature and acclimation were going to happen.
Now, I have a client who is going to run a marathon in FL in January. Since it will be much, much cooler (at least it normally is) where he lives, I will have him either "overdress" for easy runs during his taper or run inside on a treadmill. If I had access to a lab that I could get him into, I would monitor a few things, but as it is, we will go with the best guess.
I swear if I ever run Boston again, I will run on the treadmill or in lots of clothes since my luck lately has been that it will be hot for any marathon I enter. Sorry for all those running Chicago this year.