I used to not care about achievements either until one day I got tired of spending my money on games and never finishing them. So now I make it a point to 100% every game I get from now on, and at least the majority of the achievements, if they exist. Plus, having additional challenges beyond the scope of the game is nice too.
But screw this Underdog achievement though. Too luck based and I care more about my sanity at the end of the day. Don't see myself ever getting it unless some freak miracle happens while getting the 300 laps in multiplayer achievement.