My ol' Shovel used to foul plugs in city traffic, but I would get out on the road and they would clear up. But in answer to question, yeah, I carry a spare set...
Yes. When I rode a shovel, I carried and needed plugs. Old habits are hard to break so I still carry them, but have never needed them for an evo or a twinkie.
As an old habit my "kit" still contains a master link (now that's a conversation piece), plugs, some bailin' wire and duct tape and a few other items - - I've put on a few miles on the ole RKC without a hitch (knock on wood) - but on my Sturgis trip last year I got to use the small hose I carry to siphon gas from my tank to help out a brother rider along the way. Better to be prepared!