Of course the feature would be optional and could be turned off through the columns menu.
It would be very helpful for me though.
Let's say for example I have 5 lightning bolts in my inventory and in one deck I have 4 lightning bolts.
The columns in inventory would now be 5 for inventory 0 for tradelist 0 for wishlist.
The suggestion is to add a 4th column which could either be "available" or "deck". If it was available then in this example the column would list 1 because I only have 1 available to put in a deck. If the column were deck it would show 4 because I have 4 being used in decks.
The second method is futile. I mean when building a deck, you do have the number of cards in your inventory and in your current deck available side by side. You can do the math
This doesn't quite work because, going back to my example, I'm building a second deck so it still shows 5 in my inventory even though I don't have 5 available to use. I don't know about you but I have many many decks and I dont want to go through each one to search for the cards I need to take out and put in another deck.
So although it may not be useful to everyone, as an optional feature it would benefit a lot of people.