Question: Card sleeves are probably be a no-brainer for board game collectors who want to keep their games in pristine condition for as long as possible, or for CCGs where an individual card may be very difficulty to replace. 

For non-collectors playing games like Agricola, 7 Wonders, Mage Knight, Pandemic, etc, when does it make sense to use card sleeves and when doesn't it make sense?

Answer: 
There are a few situations where you want to consider sleeving your cards

* You want to prevent marking caused by uneven use (ever play Euchre with a normal deck of cards?)
* You want to prevent marking caused by tears, folds, scratches and nicks (especially in games with a lot of shuffling or playing cards repeatedly)
* You want to protect the value of the cards (for collectible card games, such as *MtG*)
* You want to protect the condition of the cards (because you want them to remain in a nice state)
* The cards are difficult or expensive to replace

And a few reasons you might want to not sleeve your cards

* They will not fit nicely into the original packaging you have with sleeves
* The cards no longer stack nicely with sleeves
* It doesn't have much impact (if any) on the game if cards become marked (resource cards in *Settlers of Catan*, for example)
* There is information on both sides of the card that needs to remain visible (you can, however, buy transparent sleeves)
* The cards are easily replaceable (such as a normal deck of playing cards)
* And, of course, sleeves cost money

So, in short, it's really a matter of personal preference. 

I sleeve my cards when playing *MtG* as some of the cards are valuable and marking can be an issue. 

I don't sleeve my cards for *Arkham Horror* as (although they are very worn) it isn't very important to keep information secret. The cards also come in various sizes and sleeving them all would be annoying.

For another game, *Citadels*, some information is very important to keep secret (roles) so I sleeve these cards to prevent marking, but other cards (buildings) are not as important to keep secret so I do not bother sleeving them. The same idea applies with *Battlestar Galactica*.
Another, not obvious reason when you want to sleeve your cards, is when you suspect your cards could be easily damaged.

It can happen, when you are going to play some multiplayer games like Dixit Odyssey on your lake holiday with a bit drunk friends, when everyone probably would have their hands dirty, not to mention a few times sleeves saved my cards when someone fall over candles, or some glasses with juice or coke. Then, you are really happy that you bought sleeves.