Lack of counter-party risk. There are a variety of mostly technical reasons why it is desireable that fees be paid in a peer-to-peer issued currency vs a central-party issued currency. For example, if there are multiple competing currencies in use (e.g. usdcoin and eurocoin, or more likely bitstampusd vs bitreserveusd), then a p2p currency like bitcoin provides a neutral platform for setting fee policy.