If I were to say "the tress with their battered barks" I would definitely use plural. I guess it just depends on the individual.
I always thought calling the trees dusty was just trying to create an image--though now that I'm on the spot, I'm not sure what image exactly. Trees that are ignored by all of the busy people? I dunno... :P But it doesn't bother me.
I think I prefer the second ("barks") because I associate "barks" with "trunks". Presumably, the trees were battered in a series of incidents, so my mind doesn't leap to the numberless meaning of the noun.
Contrast with the following:
"Pine trees have a rough bark; aspen trees have a smooth bark."
(It may also be regional, for all I know. My speech is a weird mix of Midwestern and Southern.)
Eric, "trees with their battered bark" does sound as if the battering were the result of one incident, rather than thousands. And FTR, I don't have a problem with "dusty trees", but I have always wondered about it. NYC gets plenty of rainfall; I don't know why the trees are so dusty.
Now here in the desert, we know what "dusty trees" are. LOL.
Gaveston, all of that points towards bark being a count noun for you - count nouns can be pluralized and can follow the indefinite article. (Contrast with a mass noun like air that usually resists these things.) There may just be individual differences on this, as Eric says, which would explain why some of us puzzle at the line from Company and others see no issue at all.