It would be great if folders could be used for more features. If a folder could aggregate the cards contained in the decks within it, you could do things like set folder-level settings for whether you are sharing cards between decks, and see things like how many of each card is used within a folder. An example would be that I have a hierarchy of:

Tournament Decks folder
Legacy folder
deck 1
deck 2
Modern folder
deck 3
deck 4

Then for the Modern and Legacy folders I could set the option to not share cards between decks, but within Tournament Decks I could say to share cards between decks. This would effectively make sure that I had enough copies of each card for all Modern decks or all Legacy decks, but I'm not going to be playing Legacy and Modern at the same time, so I'm OK with sharing cards between formats. This would also provide a nice way to view the list of all cards in my Modern decks by selecting the Modern folder, or all cards in tournament decks by selecting the "Tournament Decks" folder.