Yes tyre weld ruins the tyre so ive always been told,so you cost yourself a fortune for the sake of a possible simple repair.
Also,if you rely on the AA towing you home or whatever,just say you're on your way to a big meet or something.For the sake of carrying a spare you've now got to go home or go to the nearest place to get a new tyre and all the hassle that goes with it.I'd at least carry a space saver,and to be honest all this weightsaving malarkey is well and good if you're stripping your car out,but just removing the spare to gain any handling or performance gain is a waste of time IMHO unless you go the whole nine yards.
The runflat issue,if you have no sensor,is it not possible if the tyre is slowly deflating itself you may not actually be totally aware that its doing it at the time.Never had them so dont know for sure,but seems a risk there to me.