You make very good points Snuffy, but sort of skirted my point.
People do look for shortcuts. Many times I read about how someone has a new gun and wants a good load or max load and goes to the max load with a slight reduction as a place to start, effectively working the load down and not up. This is not good practice and has certainly been the seed which has lead to the KB of guns, I'm sure. There are manufacturing tolerances in both Guns and Ammo and if one happened to get a bad combination while toying near max, things could get exciting in a hurry. With published or pet loads, I start at the start load, while keeping in mind that around XX grains is probably where things are going to come together nicely.
Because of the fact that it is nigh impossible to exactly duplicate a certain published or pet load, working it up is mandatory. Not having the exact components and expecting the same results as another is wishful thinking. And as Brian noted, to introduce other variable into the equation such as custom chambers & twist rates will widen the gap of reasonable expectation even further.
But for many standard production guns, it's no miracle for a pet load to shoot good in more than one gun. POI may be a little different but grouping will be on par. I've had very good luck with my let loads (usually cast) shooting as good in other peoples (standard) guns as they do mine. To a certain extent even different boolits but of the same weight. Good ammo is good ammo.