Does plugging in an iPhone/iPad make a difference to its battery lifespan?
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.