Does plugging in an iPhone/iPad make a difference to its battery lifespan? I've been led to believe there's a difference between how a MacBook and an iPhone/iPad consume power: when a MacBook with a fully charged battery is plugged in, its components draw power directly from the charger while when an iPhone/iPad with a fully charged battery is plugged in, its components still draw power from the battery which is frequently recharged to keep it at 100%. Hence, it doesn't make a difference to the battery's overall lifespan whether the iPhone/iPad is plugged in while you use the device. 

Or in other words, if I'm using an iPhone/iPad and am not worried about battery life, there's no point in plugging it in anyway in the hope of prolonging battery lifespan?

Is this correct or have I been misinformed?
Deeper discharge/charge cycles shorten an Li-Ion battery's lifespan faster that shallow ones.  (e.g. One 100% cycle is more wearing that four 25% cycles, etc.)  Therefore, keeping an iPad or iPhone on a charger or always topped off between short uses will tend to lengthen the battery's total lifespan by reducing the number of deeper cycles per annum, all other thing being equal (such as total time of use).

"Calibrating" has almost nothing to do with the battery lifespan.  It's mostly needed to help the battery gauge icon behave more accurately.  Once or twice a year is probably good enough for that purpose.