A 10K service interval on modern sparks plugs is still a waste of time and money even if you only have to pay for the shipping. My original 2007 Denso iridium's have a ton of miles and still don't need to be changed according to the FSM.
Not on a tuned VW with 2x original power. Again...read the comments above for my situation. 10-15k is the typical recommendation from the tuners. $40 once a year to avoid misfires and running like crap..yes please but if that's your jam good luck to you. On a stock car? 40k is VW's interval.
A few spares makes sense, but spark plugs are typically not a frequent service item so I don't understand why anyone (other than a parts store) would have a stash. Someone enlighten me please.