My take on the role of excercise bikes in the Fifteen Million Merits episode is that they're more of an abstract representation of what is most likely a meaningless job.
It's possible that the bikes are used to generate electricity, but that would be very inefficient and, also, not a very creative interpretation. Maybe they're part of a fitness experiment - the participants have an exercise regiment that they keep and are being monitored for the effects the excercise has on them. In return, they are provided with lodgings and food. After all, people have been paid for weirder things in modern times. I can think of a few more scenarios where the bikes are a part of some data gathering/social experiment thing.
Overall, I think, Black Mirror has a trait that distinguishes good fiction from bad - it manages in each episode to create a world that is abstract and unfamiliar enough to play by its own rules, but at the same time, allows parrallels to be drawn between the fictional world and our reality.
It's possible that the bikes are used to generate electricity, but that would be very inefficient and, also, not a very creative interpretation. Maybe they're part of a fitness experiment - the participants have an exercise regiment that they keep and are being monitored for the effects the excercise has on them. In return, they are provided with lodgings and food. After all, people have been paid for weirder things in modern times. I can think of a few more scenarios where the bikes are a part of some data gathering/social experiment thing.
Overall, I think, Black Mirror has a trait that distinguishes good fiction from bad - it manages in each episode to create a world that is abstract and unfamiliar enough to play by its own rules, but at the same time, allows parrallels to be drawn between the fictional world and our reality.