Does dry-brushing really do anything...

I’ve pretty much reached the same conclusion- that they don’t do much but push the dust around. The techniques for the Decca/Deema/Audioquest seem to involve pushing the brush sideways or at an angle --one person suggested that contact with the spindle of the record player made a difference- but I find them, and the Hunt EDA (using the scoop technique) largely ineffective (although the latter will give you a nice line of dust). Somebody else suggested that the older brushes were better, so I pulled out an old West German Deema I had, and I do use it occasionally table side. I’ve also tried that "big brush" with grounding wire- meh. I use it to dust the platform on which the table sits, but it doesn’t seem to do much for records. :)
I clean the records really well, using a Monks and a KL, and resleeve- and like the OP, if a record seems to have collected dust after that, i will pop it back into the US for a quickie. I also agree that the brushes can create static. One of the killer brushes was the one with the active polonium element used for brushing photo lenses- man, that thing can carry a charge! (Don’t eat the polonium). I don’t have much of a static problem after cleaning and resleeving so I usually leave the record alone. I also don’t stuff the sleeved record back into the jacket, but into a poly bag that is part of a ’sandwich’ I create with multiple layers--sleeved record in the middle of the sandwich. Less friction, less static.
Remember all the various brushes, the parastat, the dust bug, the original Decca, the original Discwasher? All history.

IMO, it's more of a ritual than a necessity.  I agree that the type of dust it removes does not affect the playability, although I'm sure Doug Deacon would disagree.  Of course it is possible that a piece of crap somehow got onto the record that the brush would catch, so I guess it pays to take the five seconds and just do it. 

Depends on the weather/humidity level.

The Audioquest brush works well during the Summer when the humidity is higher, but during the Winter in the Northeast it creates static on the LP, which then adds the Zerostat to the ritual.

I stopped using a dry brush a few years ago, and instead have been using Last All Purpose Cleaner after each play.  This works well to pick up any dust before putting the LP away into its anti static sleeve.  I dispense the Last cleaner from a small spritzer bottle rather than the dropper bottle it comes in, for better control of the amount of cleaner that goes on the brush.

The old discwasher with a few drops of solution on the edge kept my records clean enough for decades that most still look and sound great.
For me, no dry cleaning process I have ever tried (Audioquest, Hunt) works as well. Ultrasonic wet is the ultimate.