Exactly my question as well. I’ve heard that once you reach a very high fitness level, long sustained periods at your Z2 can still be quite taxing. I probably won’t reach that level of fitness or high amounts of training hours to get in that situation. 
Like you, I’m curious if my 1-2 hour indoor Z2 rides would be better served at a constant power. Or maybe it doesn’t really matter as long as we’re consistently following the workouts as was mentioned earlier.
Ive done a couple recent 2 hour endurance workouts and it was 5x22 minutes alternating by 1 watt for each block.
Essentially just one giant Z2 block