From what I've been able to gather, on average you exercise about a third as hard. If it makes you bike three times more often, it's worth it. It also means you will be bothered significantly less by wind or other variability in travel time (maybe you are tired that day, for example).
Even a third might be being generous. I have never once really broken a sweat riding an eBike even in hot summer weather. I've actually had to put jackets/jumpers on to stop the wind chill on hot days - this never happens on normal bikes due to the effort exerted.
This however is precisely why I have some ebikes in addition to human steam powered ones - its so nice for commute or journeys when you want to arrive feeling refreshed but not need a shower or change of clothes.