It really depends on the weather. I always carry extra gear for if it is cold - I can throw more on. BUT
If its hot - I turn in the leathers and bust out the mesh. No, its not as protective, but what good is protection if you're so heavily distracted by how you're pouring sweat and its beading above your eyes at a red light.
I generally wear all my gear, including leather chaps and a leather jacket with my reflective mesh jacket over the leather jacket. If it gets really hot, I take off the chaps, if it gets even hotter - I take the inner leather jacket off.
If its over 90 - I've been known to leave the jacket behind. I know this isn't the safest choice obviously - but hey to each his own. If i am not comfortable - I'm distracted - which is more of a safety hazard to me than actually not wearing the jacket.
On the hottest of hot days, the jacket and chaps are the only things that come off. I still ride with the helmet, gloves, jeans, and boots covering the ankles.
Flame me, whatever - I'm not riding with 30lb of leather on a 90 degree sunny day. It just isn't practical for me.
I still stand by ATGATT, 90 degree days don't happen often here - so this is a very rare exception I am willing to make.