The problem with arbitrary goals is that they have no basis in reality. For me, an arbitrary goal suppresses my ability and desire to meet it.
I didn't even think about mileage goals until after I had an idea of how much I would actually ride. That helped me keep my goals anchored in reality. Once I had a track record, I could project it forward, then add some for incentive.
Saturday I beat my mileage for all of 2007 (4,333). I have 389 miles to my goal for this year (4,750), and 839 miles to my challenge goal (5,200) for this year. Both seem achievable, despite the latter one being somewhat arbitrary.
Cycling strictly for purposes of attaining a goal seems too much like work for me, and I refuse to hop on the bike simply for the sake of miles. Every ride has a purpose, and every ride is enjoyable. When it can't be both, I stay home.