Like a lot of things, it's a matter of degree. I propose a sliding scale of 0-10 for estimating the security of a glue job.

Zero: no glue on rim, no glue on tire. You could ride this in an emergency if you're really careful with braking and turning... pump up pressure to max.
Ten: perfect preparation, perfect glue application, perfect curing time. You could ride this in a 6-corner criterium in August. You'll probably ruin the tire if you have to take it off.

Replacing a flat on the road with a pre-glued spare would be about a 3 - good enough to finish the ride if you're not worried about another flat.
I figure my process for a new install gives me about a 7 which is safe and confidence-inspiring; more than that is just pointless overkill for my riding.

I submit that if you had a 7, then needed to replace a tire and you put on a tire that had fresh glue 6-months ago and put it on the rim you just took a tire off of (clean, garage conditions)... that'd be about a 5.