As it sits right now, a thrift store would be hard-pressed to sell a bike like that for more than $10. Keep that in mind as you decide how much fixing it up will cost. The value of the bike will still be next to nothing, even after being fixed up.
While the bike won't be worth anything to anybody else, that alone will prevent it from being stolen. Fix it up only if you (or somebody close to you) will be riding it. You won't make a penny fixing it up to sell.