Pros: it works
Cons: most brown sugar is just regular cane sugar with caramel added so you are paying more for that, more work to dissolve than corn sugar
If it is all you have (or can get easily), use it, but if you can get corn sugar I think that will be easier to use. With the amount you are adding, there will be very little effect on the flavor, some people use it in larger quantities to add some caramel flavor.
I've attempted it a couple of times, did nothing for flavor and in both instances it 2 to 3 times longer to carbonate.
Better off using corn sugar, and adding alittle molasses in the primary to add flavor. ALternately I believe some ethnic food stores carry real brown sugar, but I haven't investigated myself.