Poly count is usually more important when you're dealing with game models. In many games, there is a limit (usually arbritrary) to the number of polygons any particular model can have. To be more precise, there is a limit to the number of *triangles* in a model. Showing poly count in your viewpanel is a good way to keep your model under the limit.
However, the poly count display in Maya can be misleading. A poly is a face defined by visible edges. This can be a problem getting accurate numbers, because a poly could be a 4-sided face which is actually 2 triangles. You can try this with a simple cube. Make a cube, turn on the poly count display, and you'll see 6 polys. Triangulate the cube (Polygon > Triangulate) and you'll see the number change to 12.
For general use, most people don't need to worry too much about poly count. However, if you're working with game-related art, you'll need accurate numbers. The built in poly count display is fine as long as you remember to triangulate your model (which you'll have to do anyway). But, if you want precise triangle counts, you can use the Triangle Count HUD display menu item from AW's Bonus Game package. Or, if you really want, I can send you the actual mel script that toggles the triangle count display.