<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Analise:
Only gov't officials see the passport. Why go through the hassle which people described above of getting a new one. It isn't like you are going to give copies of it to your loved ones to put in their wallets so just keep what you have and travel to your heart's content. Live with the ugly mug shot. 
</font>
A good reason is if the current photo doesn't resemble what you look like now. Getting a recent photo that looks more like you isn't only "nice", it's a requirement.