As I said, virtually all top players have dozens of these in their cities
Nope.
Some, perhaps more than a few . . . maybe. But 'virtually all' - hardly.
The only justification for watchfires is if you are overly sensitive to the possibility of being plundered. Then, yes they can
aid in city defense. However, even with watchfires, ya can still get whupped.
And, once you reach a certain point, there is the chance that you won't really care if you are plundered. So much stuff is produced in-city, plus all the side goodies of Guild Expedition, various events, and whatever, that you really don't miss the stuff. More of an annoyance really.
Better to use whatever you have to spend on other things. And remember, 'dozens' of watchfires does take up space.
. . . mk