"Need" is an interesting concept. The cards are rarely shuffled, and little-handled. For the most part, they just sit on the display boards before being built, after which they just sit on the players' boards.
Although I cheerfully sleeve a lot of stuff, I cannot see the benefit of sleeving in this case.
Of course, it's all very personal, the answer to the OP's question is very helpful if you feel the absolute need to sleeve, but I wouldn't want anyone to read this thread and think that you *must* sleeve the cards.