I mentioned in the first Extinction Thursday why you should care about the extinction of seemingly insignificant plants. Jared Diamond puts it much better in his book Collapse: How Societies Choose to Fail or Succeed:
But biodiversity losses of small inedible species often provoke the response, “Who cares? Do you really care less for humans than for some lousy useless little fish or weed…?” This response misses the point that the entire natural world is made up of wild species providing us for free with services that can be very expensive, and in many cases impossible, for us to supply ourselves. Elimination of lots of lousy little species regularly causes big harmful consequences for humans, just as does randomly knocking out many of the lousy little rivets holding together an airplane.