Why is the Hardy- Weinberg equilibrium model useful to population genetics?

The geneticist's definition of evolution is "A change in allelic frequencies."  The H-W model lets us caluclate allelic frequencies for a particular gene to see IF a population is evolving with respect to that particular trait.  

If the conditions for maintaining a H-W equilibrium (these can be found in any Biology text) ARE met, then frequencies for a particular set of alleles will not change over time, and, therefore, evolution is not occurring.  However, the condition requiring no Natural Selection is NEVER met in a wild population, so there is (technically) no possibility for maintaining H-W equiilibrium.