Thank you for the answer to my question. I did not expect to get a response on the merits of declining a reward, but since I did, I would like to ask a few follow up questions:
-Are there ever times in the game that you would purposefully decline to roll on a boon because you would not want to acquire it? If so, you are most likely weighing the importance of getting an extra hit point over diluting your card pool. This same decision making process should be engaged when deciding whether or not to take a card feat. Note that card pool composition matters more when you have a character that recycles their discard pile into their deck as part of normal play or has a card cycling mechanic that cares about card types (such as wizards with spells).
-Do you think that there are a few cards in each card type for a given character deck that are significantly stronger than the other choices in the character deck for any given character level, or do you think the cards are roughly the same in effectiveness?
-If you do believe that a few cards in each card type category in your card pool shine above the others, why would you want to take card feats to expand your deck in order to accomodate "weaker" cards in that category? Your deck should theoretically contain the best cards that you currently have access to. Increasing the number of cards in your deck forces you to put cards in your deck that you previously decided were not among best choices available to you.
It is understandable to want to add cards to your deck if:
-you want to increase the chances of drawing a certain card type
-you desire to add to the number of cards of a specific card type in order to shore up a specific weakness in your character's deck
-you think that adding cards to your deck is advisable due to the hitpoint mechanic (this factor becomes more important if large amounts or frequent occurences of unavoidable damage and/or forced card burying/banishing is prevalent in the game)
Otherwise, it appears to me that adding cards to your deck actually hurts its efficacy.
I should add that there is another reason to add cards to your deck, whether or not it is the best decision mechanically: if adding a new card slot to your deck makes you enjoy the play experience more, that should be done; optimising fun is the most important type of optimisation of all.