Except that if you fail to include a transfer fee big enough, your transfer may take weeks, or never even get confirmed at all. Case in point: I've been trying to actually buy something with my BTC in the last few days, but because my wallet is an older version or something (with a small default transfer fee), my transfers are seen by just one peer and then are not confirmed. I dunno, last time , 2 years ago, when I paid / transfered bitcoins, it took 10 mins max. I guess the mining business has increased or changed since then.