"My" mechanic at the local shop says it has been decades since he has patched a tire from the inside. Besides the plugs holding fine, if the hole/damage is too big for a plug and possible a patch would work, they go ahead and replace the tire. Liability / insurance reasons mostly.
I actually don't use cement/glue on my plugs - I found that many times it can make the plug too slippery. "Fresh" plugs are gooey enough and hold just fine... I have found that plugs can dry out and be worthless. I usually don't find that out until I really need one........