[Joe Sr.]

Effects of pitching rate aside, I'll go back to post #2 and agree with bayareabrewer.

On the homebrew level, it's pretty hard to overpitch.  You need to do something deliberate such as pitching an entire yeast cake, growing a huge starter, or pitching many packs of yeast.

As far as does it matter?  As Keith pointed out, pitching rate matters most with respect to consistency.  All other things being equal, you'll need to pitch a similar amount of yeast to consistently repeat the same beer.

At the homebrew level, such consistency isn't always an issue.  It is for some of us.  Not all of us.

[brewmaster]

Overpitching can result in yeast autolysis but is most evident if you are reusing your pitching yeast.  If reusing yeast and overpitching in each batch there will be very few new generations of yeast created, and the older generations of yeast will eventually become tired and sluggish.