Mostly, we train because we think there's a relationship between dose and response. Coaches prescribe certain kinds of workouts in order to make sure their clients get the intended dose. Highly structured indoor training plans have convinced many riders that precision in structured dose is what's important, even more so than volume of dose. So outdoor training worries them because it's hard for them to follow the structure. Highly structured sessions are the result of the lack of variability in indoor trainers (erg mode is the end point of lack of variability). Hyper-structure is there to make up for the deficiencies of indoor training. You don't need to impose artificial structure outdoors because there's typically enough variability from riding in the real world on real roads.