It is completely fine to leave it plugged in AFAIK but the key difference here is about the lack of use (see musings below).
As jjrandorin implies the car won’t constantly micro charge (e.g. from 89.5% to 90%) but will wait until it has dropped a few percent then charge back up to your set limit (”87 to 90”). These kind of very shallow partial cycles aren‘t very harmful to lithium ion batteries compared to deeper discharges (other than the fact that using/cycling the battery at all puts wear on it, as does age).
The “charge to 90% and then set 70% and allow to fall, then repeat” strategy is likely aiming at getting to 90% fairly often (possibly for something like cell balancing, keeping the BMS estimate accurate) but not staying sat permanently at “high” SoC all the time due to lack of use mentioned. Hence allowing charge to fall gently down to 70% allows the battery to spend more time at a lower SOC which is nicer for battery life but still means you have a useful amount of charge at all times in case you do need it. This manually optimised pattern may be slightly better for the battery of a car that doesn’t see much use at the cost of a little effort but I suspect the difference is still likely to be small over the car’s lifetime so I wouldn’t sweat it very much either way.
[note: If the car is used most days then leaving it plugged in with a scheduled charge and the limit fixed makes most sense - the car won‘t spend long at a high SoC anyhow before it gets used and will sit at a lower SoC the remainder of the day until it’s time to charge again for the next day’s use. This is what I do - mostly because I have very cheap overnight electricity rather than worrying about the battery per se.]
In my specific case, I am not trying to balance the battery or anything (although I agree with your explanation here). In my case, I charge to 90% simply because thats what I have done since I picked up the car 2.5 years ago. My normal commute to work is 80 real miles round trip, and I plug in every time my car hits my garage, no matter how far I go.
Because I work a lot from home now (for the same reason many others do, as I mentioned), I can go days without driving, instead of driving 80 real miles a day. I have comparatively low battery degradation for my model 3, compared to some others who micro manage it.
I only change the slider to attempt to minimize somewhat contactor opening and closing, since I am not going anywhere.