I will give general reaction, followed by factions in seperate posts (I won't remember all cards otherwise).
First there is a difference between number of cards I "need" and number I take. I might take more than I need either to improve chances of drawing them, or to guard against their loss.
Second, I might choose a card for multiple purposes, some very unusual. I can certainly see choosing sea dragon for its 4 strength independent of its spell draw power. In fact, I would probably choose 4 sea dragons over 3 sea dragons and an underdark worm just because the worm's 1 extra health does not offset the chance of retrieving another spell. I might play forest canopy, not for its power, but because it is a zero cost way to keep a dreadmarsh plague of mine out of a particular lane.