I had about 5 years and 30k on my current plugs and replaced them along with wires and intake manifold. I ended up having a few that were 'white' pretty much frosted and some normal, and one that seemed oil fouled, but I probably spilled oil on the block and it went into the boot at some point.
Now while I was doing that manifold and chasing a cel for misfire, I could have trouble shot the heck out of the issue and than ordered the plugs and wires, or I could replace them, do no trouble shooting and know I have new parts, 0 aged, and 0 miles on them. I used MC plats and MC wires to replace MC plats and off brand wires.
When I did the job 5 years ago I noticed the plugs were same color coming out as going in. So maybe the previous owner did them recently, or maybe the engine was running better back than and there were less vacuum leak issues and seafoam treatments and no K&N air filter.
Regardless.... $35 that saves debate and gives piece of mind along with knowledge of resetting the clock on your plugs life is worth the $35 imo.
Now if there was no reason to pull the plugs and everything was running fine, yeah I would have kept using them until there was an issue than replaced them, but because I was already being told 'pull them and inspect them for coolant' it was kinda a no brainer to replace them once I had pinpointed the root cause of the problem and fixed it.
They probably still had life in them, but am I going to stake my engines life (and how it runs) on probably? They were most likely damaged from one or more of the issues I mentioned over the years and running a damaged part, especially after I removed it and looked at it in my hand, seems stupid. Especially when the part is cheap, and installation is reversal of removal. It's not like I won't torque down the old part, but would torque down the new part.
So yeah I use MC plats because I hate doing the job twice and I hate inspecting the part to see if it's ok or not.
Also someone mentioned that double plats are the way to go...... idk if it's true but my buddy told me not to do bosch double or quads even before I got here. I will quote him now because it makes sense.
'They claim to give you more spark and burn more fuel thus increasing fuel efficiency and hp, as if a single spark will not ignite the same fuel as much. if a single spark is not igniting your fuel you have a big problem besides debating weather or not to use double or quads. On top of that they pretty much want you to believe that your coil packs and wires are capable of a lot more spark and somehow by giving them more points to release that spark your making more spark..... No. Your taking the same electrical charge to make one spark, dividing it by 4 to make 4 lesser sparks... So yeah those 4 sparks together are probably as strong as your single, only difference is if one goes bad you have 3 more to take over. So yes you will not know you have engine problems as soon as they occur'
Made sense to me, not designed for our cars so why would they improve anything when the amount being put in is constant. Now if you got a tune and someone made sure more electricity was flowing to the plugs maybe they would work.
You could have replaced all your plugs and wires in the time it took to read this dribble :p.