Ghostcrawler took to the official forums
to address the lack of spirit cloth gear that some players have experienced in the Mists of Pandaria
beta dungeons currently available for testing. Spirit gear for pure healers is necessary to retain mana regeneration levels and output as content increases in difficulty. With a lack of spirit gear, some healers found their jobs more difficult. Ghostcrawler confirmed that the level-up dungeons found throughout the 85 to 90 content cannot be over-challenging due to the nature of their place in the progression.
The meat of the answer is that as you approach endgame content at level 90, more gear appears with endgame stats on it -- dodge, parry, and block for tanks, spirit for healers, and so on. When that gear becomes important is when the stats start appearing. Until then, pop some reforge, check reputation vendors and other sources of level-up gear for spirit stuff, and mish-mash together a set of decent healer gear. (Or, at the end of the day, you're just really unlucky with drops.)