One thing to remember about plugs. They need to be driven on in order for the vulcanizing adhesive to warm up and seal properly. If you just plug it and use it as a spare it may never seal properly. If you want to use it as a spare then drive on it a awhile first then switch it out.
I have plugged 1000's of tires in my life time. They work extremely well when properly install. But a lot of people out there can screw up the simplest things.
skeeter