I assess how well the active recovery workout went, and how my ability to perform a similar endurance workout only 2 days after the first one went as well.
A power meter is excellent biofeedback for such workouts; while time alone is sufficient to determine relative effort level on a fixed ascent, only a power meter will tell you how you are doing in rolling terrain and with wind, etc. And a power meter also takes into account any extra weight (water or whatever), so it remains objective no matter what— neither time nor heart rate can do the same.
Just as key, a power meter keeps me motivated and focused on the workout in a totally objective way; I can take its input and make a rational choice on what effort level to maintain. It’s the most rigorous “riding buddy” you can have.
A friend of mine (who has never used a power meter) really doesn’t understand the incredible feedback loop of a power meter; he is still thinking in terms of “faster riding buddies” and similar non-objective approaches to training. While friendly competition with others is certainly a good thing to mix into the routine, it’s not a rigorous or optimal way to achieve training goals. And since I’m highly self motivated and ride on my own schedule, trying to find just the right riding partner for the workout goal for a particular day is almost impossible.