I recently started using a MyWhoosh goal-based daily workout plan. Interestingly, the recovery days are not strictly Zone 1 or Zone 2 or a mix of the two. The recovery days mostly include intervals in Zone 3 or even Zone 4 or 5 (with the duration riding in each of the harder zones being progressively shorter, of course). I've used Xert and Trainer Road, too, and they do the same mix of zones for recovery days.

So the coaches who plot out these plans for those companies seem to believe that unvarying adherence to Zone 2 is not an absolute requirement for achieving improved fitness. (FWIW, I've been doing both of the recommended workouts per day, totaling close to 14 hours per week of indoor riding, so the mixed-zone plans apparently don't represent compromises meant for the benefit of time-crunched riders.)