Some very interesting and open minded opinions above.
Just a few ramblings...
The first thing that I think of when ebikes are mentioned is the sale pitch given by seemingly all manufacturers, which goes along the lines of "Arrive at work sweat free and relaxed" A statement that if taken in the literal sense, would mean that zero effort is required, which probably does nothing to promote ebikes as a fitness and exercise device.
My second thought is one that Kernow stated. "you can still be lazy if you want to be , but your battery won’t last long" Those few words often spring to mind when I read comments from eMTB owners riding at the maximum power setting everywhere. Okay, they could be traveling at a faster pace and pedaling at a higher cadence as well, but that isn't how I perceive their riding to be. I perceive it as just wanting an easy life.
Without doubt, a pedelec of any from can be a brilliant aid to gaining fitness, but it is very subjective to how it is being used, although even on the highest power settings with least pedal resistance, you arguably must be getting some form of fitness benefit, and deffinately more than you would sitting in front of a television etc for an evening.
I have no idea what my heart rate is when riding either an assisted or non assisted bike, but I do know that for me there is only two points of relevance, and it isn't fitness based. My two points are mental attitude and rhythm or pattern of breathing. If I head out and get either of those two wrong, then I'm not going anywhere, and it is all too easy to blame that on fitness.
It probably took me a good two years to sort my breathing out, but with that one aspect achieved, the leap forward in what I could achieve was massive. It takes me about three miles on an unassisted bike to settle into things, but once at that point all is good.
Last night I was out with my regular riding buddy, riding my unassisted Giant XCT Advanced plus with its 11-42 casssete and 32t front chain ring and just felt unstoppable. I had a really tough and physical day at work, and really didn't want to go out, but weirdly even the first climb from my house felt effortless, and I was away. I was being smooth with my pedal cadence, breathing properly, and just had the right mental attitude.
My fitness has been pretty up and down though this year, but that has been due to chronic knee pain and swelling. Thankfully though, I took up a low impact fitness class (Kangoo Jumps) that has helped sustain and increase my fitness, kept weight in check, not that I have ever suffered with that, but most importantly of all, has trained me to breath properly, back to that one again.