If you don't need one, you don't need one, period. That's why I chose the system I picked - because it comes off the bike and I can ride analog with less than 10 seconds prep. I can also have it on the bike but not even turned on and, since it only weighs a total of 4.5lb with zero drag, there's no penalty for just leaving it on. Then it's there if I decide to tackle a hilly route, or if the big Bay headwinds come up, or if I didn't sleep well the night before so I poop out. It also has 9 levels of assist, and I can get just as much exercise with level 1 as I do riding analog - by choosing a route with lots of ups and downs and doubling the length I normally ride - twice as quickly.
But, yeah, if you ride a normal route with no motor, you can't just add the motor and expect the same effect in terms of fitness. You have to ride longer and take on bigger grades. Now for some folks, that's a reason to get an e-bike. You WANT to go longer distances and over more difficult terrain than you can manage with your analog bike.
So - if you would like to double your distance, while not doubling the time, you might find one makes sense for you. And if you pick more challenging routes, you can definitely get that fitness experience. Just depends on what you want. If you do the same ride but with a motor, it will be quicker and that's all.