Pedaling at lower cadences with higher resistance develops leg strength the same way lifting a heavier weight in the gym increases muscular strength. But just like you have to be careful about how much weight you lift in the gym, especially if you have any joint issues, you must be careful with resistance on a stationary bike. This type of high-resistance cycling training is often referred to as strength endurance.
Outdoors, cyclists are encouraged to climb at cadences above 70 rpm, but that depends entirely on the gearing on your bike, the steepness of the hill, and, of course, your fitness. You may be in your lowest gear and encounter a grade that requires you to pedal slower, sometimes even below 60 rpm. If this happened too often, I would suggest you install lower gears on the bike. In fact, that’s something I did a few years ago on my own bike and my knees have been much happier! (I live high in the Rocky Mountains, surrounded by long, hard climbs.)
Still, it’s not unreasonable or dangerous to pedal slowly on really tough grades, as some programs will have you believe. It’s only “dangerous” if you go TOO slow due to massive resistance, overdo it, ride with poor form, don’t prepare progressively for high resistance, or if you ignore injuries or weaknesses that would preclude you from it. You can use a low-cadence, high-resistance profile as a training session to focus on leg strengthening. Just don’t do it often or for extended periods—maybe once or twice a month, and only when you have participants who you know have been riding long enough with good form and have adequate leg strength.
Just know that some non-cyclist participants who don’t tend to train with enough resistance might be at risk. You’ll need to provide modifications for those in your class who have any injuries or weaknesses.
What is “too slow”? That’s one of those gray-area questions, as it always depends on the person. While I would still use 60 rpm as the average cadence floor for most indoor riders most of the time, it’s OK to occasionally ride in the mid-50s rpm for stronger, more seasoned riders.
This is how I introduce a strength endurance profile: