I’m not certain that leaving your phone plugged in will reduce the number of charge cycles. When you use a smartphone that is plugged in, I believe you still draw power from the battery even if it’s plugged in. That is, you pull power from the battery and then the battery recharges instantly (or very quickly). However, you’re still using battery cycles. Furthermore, because your battery is always discharging and charging from the 100% point, that’s harder on the battery than if it was between 20 and 80%.

Yes, it would be handy if there was the option to stop charging at a certain level.