Go on any site dedicated to cars running 40s and 45s, and you'll read plenty about blowouts and tire failures. No, it's not the end of the world to have them, but the risk shoots up dramatically as you reduce the sidewall profile. In some locations (like NYC), the road conditions are just too unfavorable. And folks driving on 40s also tend to drive faster. Not a good combination when a blowout happens.
I just sold my spare set of PZeros above to a guy who just had a blowout on one.