Always use it and I cook on a concrete porch.  Figure it's there so why not.  Never have any air-flow problems.  FWIW-

I use it, and always have - even on our patio at the old house.  I'm Egging on a deck at the new house with lots of trees around it, so not taking any chances.

I started using it last summer after I stepped on a hot ember barefoot that shot out on my concrete patio. 
Somehow it doesn't seem like a bother to make sure it is closed anymore.

OhioEgger said:

I have always used it. Started out on a wood deck, but moved to a concrete patio a couple of years ago, so habits die hard.

I actually find it useful. I start out with everything wide open, then when the temp starts getting up there I close the spark screen and that cuts about 50% of the airflow. As time goes by I gradually close the outer door as needed.

Yep - me too.  Never thought of it as spark catcher, more a filter.

Always!

Always... I am on concrete with wood table

I use mine. 

Hungry Joe said:

I started using it last summer after I stepped on a hot ember barefoot that shot out on my concrete patio. 

I did the same thing but when I was transferring from a Chimney.

I use mine every time but the bottom half of the screen is shot and I never remember to replace it.

I have a wood table and a wood deck, I always use it.

Always.

I used it, but my screen broke.  I recently removed the screen insert when I bought a much stronger insert that had many small holes drilled in it rather than have a screen.  I can't insert the new piece, because it's being blocked by the nest.  I'll need to raise it out of the nest, so that I can place the insert.  I'll get to it when I get to it.

I used it until it disintegrated.

I ALWAYS use mine, even though the egg is on a concrete patio. It is still within sparking distance of my garage. As I understand the story, @Nature Boy (Dizzy Pig) lost his home because he was egging in his driveway, 10-15' away from his open garage door. This was either before they had screens or he simply wasn't using it.

Also, there's the sparks on bare legs, pets and children argument.

Why would you not use it. Yeah, there's a considerable airflow difference and therefore max temp is reduced, but I can get to 900° with mine closed. How hot do you NEED?!

DoubleEgger said:

I wonder why KJ didn't put one on the Jr. Seems like the insurance companies would have made them put a screen on it. 

Why? Do all the other kamado makers have screens on their models?

I know Primo doesn't, not sure about all the others.

I guess it happens but I've never seen any sparks shoot out of the bottom of my KJ Jr. And that is with a KAB which I have no idea if that would make it more or less likely to happen.