I was always under the impression that your road side plug repair you push in, is just a temporary measure. As it could easily pop back out, leak air etc. To repair it properly it needs to be plugged from inside with a mushroom type plug glued down on the inside which has no chance of popping out.
TBH though, whenever I've had punctures, it's when the tyre is getting on a bit, tread depth getting close and so on, so I've always just stuck a new tyre on. Always deemed it a false economy to compromise grip for the sake of eeking out the last mm of tread...