Been spraying off and on for about 15 years and honestly the promoter thing depends way more on the plastic than the promoter itself. Some bare bumpers are polypropylene and nothing sticks to that without help, but a lot of what guys call bare plastic these days already has a factory primer or e-coat on it, and scuff plus wax and grease remover is plenty for that. Six years with no peel sounds believable if he's working mostly newer OEM bumpers and prepping right, but I'd want to know what he's spraying and whether he's doing raw aftermarket stuff too. Your mileage may vary by product line, take it with a grain of salt, but I wouldn't throw out the promoter on unknown plastic just because one guy in Reno got away without it.

Man, that's the kind of trick that works till it doesn't.