Yeah, I don't' want to banter the screen thing too much, but remember 99% of masonry fireplaces have no screen and PolarBear has a steel roof so there is zero chance he is going to set his roof on fire.
And this is an honest question with no sarcasm or derision intended, but can you honestly say you have never ever had to clean your cap screen since burning "good wood"? Because if your answer is no, then I'll admit that I have been doing something horribly wrong for many years.
One more thing, and this is just my opinion. I think screens give many the warm and fuzzys on safety, but they cause more dangerous problems with those who burn marginal wood. They are a designed choke point by not allowing those un-burnt particles to escape the flue which & once severely restricted accelerate the accumulation of flue creosote. All of us should be burning good wood, but it just does not always happen.
Yes, I can honestly say that other than the first year I burned this set up, I have never had a problem with the cap clogging. Now the first year, I did have to clean it mid season, as it was clogged badly. I was also burning wood that was cut the same year, and split a damn near just before going into the stove. It was dead standing oak, all dry outside and checked, but no where near dry enough. I knew it, and could easily tell by the weight of the splits as I picked them up. But alas, it was the only wood I had, so I burned it. At the end of the season, it was about 1/2 clogged again. Also due to the same wood.
Since, I have checked, but never had to clean the cap mid season that I remember. Somewhere on here are photos of what the cap looks like after a season of burning, prior to pre-season cleaning for the next season.
So short answer, since I have been burning truly dry wood, no I have not had cap screen clogging issues, and usually the entire system when cleaned, is about a mason jar worth of coffee grind looking soot. Sometimes less than that depending on what wood I burned that season.
There will be a minor thin coat of creo on the screen at times when cleaning for the season, usually about the same as is on the underside of the cap etc. Not clogging.
The one issue the cap screen has recently, is it is rotted very thin, and will be replaced after this season.
I found some 3/4" opening galv screening, and will be making a new one to fit.
Just a side note, the roof is not always the issue of embers & flyash, but more the surrounding area, woods, ground covered in leaves etc. Unless the roof is made of cedar shakes or similar, I don't see many roofs having an issue of embers, fly ash etc. If the roof is cedar, then yes, embers should be a concern kept in mind. Nothing goes up like some real old, dry cedar.