I can't talk for the Verado, as I am at the other end of the "tech spectrum", but plugs' insulation does degrade - I once meggered a set of plugs out my lethargic 2-Stroke Yam 55 - one failed Patt test as essentially short circuit, the other scraped through.
With new plus the engine sang.
I guess if you buy the Mercury approved (i.e their part number) plugs then they will degrade at a known rate and you won't loose performance etc. if you change them after 300 hrs.
Granted that is only 12.5 days (continuous)
so yes, I see why you ask.