There's a glitch right now in the client to include a lower than normal fee.  Here's a slightly problem though.  I dunno how many transactions can be put in a block and how many a pool can handle but what if it gets to the point where if you don't add a fee, your transaction could be put on hold indefinitely?  Like it could take weeks or months to go through because nobody's picking it up and it's getting swamped by other transactions?