This isn't really even all that important. Rebuilding atomizer heads is about convenience and saving money.
Sure it's convienant, takes maybe 5 minutes, and never need to order new heads..Sure, it's saving money, costs just pennies. But it's more than just these 2 factors.........
I like a 1.4 ohm coil, you can't buy these.
I prefer the taste, as do you, of cotton. Again, its not available.
I'm on a preorder for ribbon kamthal, everything I've read indicates it's superior. Again, not available on factory heads.
Who knows what contaminants are on a purchased wick and coil. I'm in control of mine.
If I want to experiment using a 2,3, or 4mm wick, even double wicks, and customize it to the juices viscosity, no problem, give me a few minutes. Can't do that with a 5 pack from your vendor.
I DIY on juices too, that too is convienant and saves money. But exactly like rebuilding wick & wire, it's offers so much more than just these 2 factors.