Probably the idea of rating posts came up at the time when there were very few users of this forum. Then the idea may have been to rate each & every post. May be bcoz of this, the Ratings vary from 1 - 5.
If an author posts something & didnt get a proper reply, he would wait a little more time to see an exact answer or solution. Instead, rating of 1 or 2 is not his idea of telling people that he's expecting more on the solution.
Ratings of 1 & 2, I believe have no significance. When someone who replied to the post gets a rating of 1 or 2, the person may feel dejected & may not bother to reply back more precisely. Also, this just spoils the average rating of a Netpro. Taking note of the fact that nobody enjoys his total score going up by 1 or 2 points. Its only 3,4 or 5 that indicates the importance of solution & also author of solution relishes it.